U.S. National Security Adviser Tom Donilon arrived in Israel over the weekend and is scheduled to meet Prime Minister Benjamin Netanyahu on Sunday evening to discuss Iran, Syria and other regional security issues. The meeting comes on the heels of remarks by U.S. Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey, who said a military strike against Iran’s nuclear facilities was “not prudent” at the moment and would destabilize the region.

In an interview set to air Sunday, but whose contents were revealed on Saturday, Dempsey told CNN’s Fareed Zakaria, “It’s not prudent at this point to decide to attack Iran,” adding, “A strike at this time would be destabilizing and wouldn’t achieve their [Israel’s] long-term objectives.”

Dempsey said that launching an attack on Iran’s nuclear facilities would not be a level-headed decision, telling Zakaria that Washington was confident the Israelis “understand our concerns,” according to a transcript of the interview quoted by Bloomberg News.

Dempsey said that increasingly tougher sanctions and international pressure meant to forestall Iran’s development of a nuclear weapon were beginning to have an effect. While estimates vary on when Iran might produce a nuclear weapon, Israel and other Western countries have accused Iran of using its nuclear program to produce an atomic bomb, a charge Tehran denies.

“We are of the opinion that Iran is a rational actor,” Dempsey told CNN. “We also know, or we believe we know, that the Iranian regime has not decided to make a nuclear weapon.”

The top U.S. general visited Israel a month ago and met with Netanyahu and senior defense officials including Defense Minister Ehud Barak and IDF Chief of Staff Lt. Gen. Benny Gantz, but refused to disclose whether an Israeli strike was imminent. “I wouldn’t suggest, sitting here today, that we’ve persuaded them that our view is the correct view and that they are acting in an ill-advised fashion,” Dempsey said in the CNN interview.

U.S. Director of National Intelligence James Clapper is also scheduled to visit Israel in the near future to discuss issues related to Iran. Meanwhile, the White House insisted that Donilon’s visit to Israel was routine, saying in a statement that it “is the latest in a series of regular, high-level consultations between the United States and Israel, consistent with our strong bilateral partnership, and part of our unshakeable commitment to Israel’s security.”

The U.S. and EU on Friday expressed cautious optimism over prospects that Iran may be willing to engage major powers in new talks over its contentious nuclear program, but underscored that any resumed negotiations must be sustained and focus on the nuclear issue.

U.S. Secretary of State Hillary Clinton and EU foreign policy chief Catherine Ashton told reporters that Iran’s recent letter to Ashton might mark a step forward.

Ashton, who handles contact with Iran on behalf of the “P5+1” group, comprising the U.S., Britain, France, Russia, China and Germany, said the letter showed “a potential possibility that Iran may be ready to start talks.”

Iran’s letter to Ashton, which was obtained by Reuters on Thursday, proposed resuming the stalled talks and said Tehran would have “new initiatives” to bring to the table. But the brief letter, which responded to a letter Ashton sent to her Iranian counterpart in October, offered no specific proposals, leaving a question mark over Tehran’s willingness to enter substantive negotiations on its nuclear work.

The White House remained “willing to engage in talks with the Iranians so long as they have a constructive approach to those negotiations, and that means a constructive approach that understands that the purpose is for Iran to live up to its international obligations and to abandon its nuclear weapons ambitions,” White House Spokesman Jay Carney told reporters aboard Air Force One on Friday.

Iranian leaders have recently acknowledged that several new rounds of sanctions are hurting the Islamic Republic’s economy.

“The sanctions have had a positive — well, they’ve had a harmful effect on the Iranian economy, a disruptive effect on the Iranian regime, and that was the intention,” said Carney. “We will continue to pressure and isolate Iran unless and until it changes its behavior.”

However, some U.S. government officials believe sanctions will not stop Iran’s nuclear program and that Washington will be forced in the end to strike Iran or expect Israel to do so. Officials in both the Pentagon and the State Department believe sanctions will fail and that their real purpose is to delay an Israeli strike, according to a report published Friday in British newspaper The Guardian.

The Guardian quoted an official knowledgeable on Middle East policy, who said, “The White House wants to see sanctions work. This is not the Bush White House. It does not need another conflict. Its problem is that the guys in Tehran are behaving like sanctions don’t matter, like their economy isn’t collapsing, like Israel isn’t going to do anything. Sanctions are all we’ve got to throw at the problem. If they fail then it’s hard to see how we don’t move to the ‘in extremis’ option.”

The White House and Israeli officials have repeatedly that all options are on the table, including the military option to thwart Iran from obtaining a nuclear weapon, but that for now diplomacy and sanctions are the focus of that effort.

If the sanctions are not effective, the relevant question is not if an attack will take place, but when. Despite U.S. Defense Secretary Leon Panetta’s recent assessment that Israel’s window of opportunity to strike Iran will be between April and June, the prevalent opinion among analysts in Washington is that it would more likely occur in September or October.

“If [U.S. President Barack] Obama reaches the conclusion that there is no other choice but to attack Iran, he will most likely not order a strike before presidential elections in November, unless it becomes urgent to do so,” The Guardian surmised. “The question is if Israel will agree to wait that long.”

Meanwhile, Defense Minister Ehud Barak on Saturday said a nuclear-armed Iran would trigger an arms race in the Middle East and that nations should impose “crippling” sanctions on Tehran to force it to give up its atomic program.

“A nuclear Iran is a threat to the whole world, not just to Israel ... Other major powers in the Middle East will have to try to reach nuclear capability, probably Saudi Arabia, probably Turkey, probably even Egypt,” Barak told reporters in Tokyo. “We have to accelerate the pace of imposing sanctions and make them crippling and consequential to such an extent that the leadership ... will be compelled to sit down and ... ask themselves, ‘Are we ready to pay the price of isolation from most, if not all, of the world?’”

Barak -- currently in Japan for talks with Prime Minister Yoshihiko Noda and ministers on issues including security in the Middle East and East Asia -- said that despite Western sanctions that have inflicted increasing damage on Iran’s oil-based economy, he had not seen any sign that Tehran was ready to give up its nuclear ambitions.

British Foreign Secretary William Hague said in an interview published Saturday that the Iranian threat has the potential to result in “a new cold war” that would be more dangerous than the one between the West and Soviet Union in the past.

Hague reiterated Britain’s position against military action and said, “We are very clear to all concerned that we are not advocating military action.” Instead of military action, Hague said he endorsed European efforts to head off any nuclear weapons program through economic and diplomatic pressure.

“We support a twin-track strategy of sanctions and pressure and negotiations on the other hand,” he said. “We are not favoring the idea of anybody attacking Iran at the moment.”

The foreign secretary did not spell out what the downsides to an attack would be, but former British ambassador to Tehran Richard Dalton told BBC television that they would likely include a drawn-out conflict, retaliatory strikes against U.S. facilities, terrorist attacks and serious disruption to world energy supplies.