The continued exchange of artillery fire between Syria and Turkey raises additional concerns that the conflict may escalate and spread to neighboring countries, U.S. Defense Secretary Leon Panetta said Saturday.

Panetta, whose comments came after meetings Saturday with Peruvian President Ollanta Humala and the country's defense ministry, said the U.S. is using its diplomatic channels to relay worries about the fighting in the hopes that it will not broaden.

His comments came on the heels of warnings from Turkey's prime minister that his country is not far from war with Syria.

Turkey and Syria traded artillery fire Saturday as rebels clashed with Syrian President Bashar al-Assad's forces near the border, heightening the fears that the crisis could erupt into a regional conflict. Turkish Prime Minister Recep Tayyip Erdoğan on Friday cautioned Damascus not to test Turkey's "limits and determination" and said Ankara was not bluffing in saying it won't tolerate such acts.

Earlier this week, Turkey's parliament approved such retaliation, expanding Turkey's response options. Turkey's leaders have reiterated that they don't want war with Syria, but another dramatic or deadly border incident could force unwanted escalation.

Turkish Foreign Minister Ahmet Davutoğlu insisted that "we haven't taken a step toward war," but Turkey's threat to fire back for each errant Syrian shell was bound to keep border tensions high. Turkey is one of Assad's harshest critics and a key supporter of Syria's opposition.

The latest Syria-Turkey crisis erupted earlier this week, after a Syrian shell killed five civilians in a Turkish border town.

The Syrian regime has apologized and tried to defuse tensions by pulling some tanks back from the border, according to a Turkish Foreign Ministry official who spoke on condition of anonymity in line with government regulations.

Meanwhile, Assad's defense minister warned Saturday that Syria's military will "crush" armed rebels as the regime shelled opposition positions in two cities and near the Lebanese border in a widening offensive.

Syria's civil war has been stalemated for months, but Syrian Defense Minister Gen. Fahd Jassem al-Freij insisted that the regime is gaining the upper hand.

The government denies it is facing a home-grown rebellion, saying it is being targeted by a foreign conspiracy against the regime's support of anti-Israeli groups.

"The most dangerous parts of the conspiracy have ... passed and the killing is on its way to decline," said Freij, who was named to the job after his predecessor was assassinated in July. He offered amnesty to rebels who repent but said those who don't "will be crushed under the feet of our soldiers."

Assad, meanwhile, made a rare public appearance Saturday to commemorate the anniversary of the outbreak of the 1973 Yom Kippur War.

Assad laid a wreath at the country's Tomb of the Unknown Soldier in Damascus, and then passed along a line of saluting army commanders in dress uniform, shaking hands with each. The ceremony, broadcast live on Syrian TV, seemed designed to show Assad remains in control.

Syria's state-run news agency SANA linked Saturday's anniversary to the current conflict, saying that in both cases "Syria is facing an enemy armed with Western and Zionist weapons."

Along with regime's new warnings to the rebels, Syrian troops backed by warplanes and combat helicopters launched attacks on rebel-held areas near the Syrian town of Quseir, close to Lebanon's Bekaa Valley, on Saturday, Lebanese security officials said.

Opposition activists also reported intense government shelling in the northern city of Aleppo, Syria's largest and a commercial hub, and in the central city of Homs.

The battlefield stalemate is most apparent in those two cities.

Syrian forces have been bombing and shelling from a distance but have been unable to dislodge opposition fighters holed up in devastated neighborhoods. Retaking Aleppo and Homs could give the regime some breathing room.

In Aleppo, government forces gained some ground in recent house-to-house combat, according to an Associated Press journalist in the city.

Amateur video posted Saturday, showed masked rebel fighters posing with assault rifles in what they said is an air defense base east of Damascus they captured earlier in the week.

The video showed captured weapons, including anti-aircraft missiles, heavy machine guns and large-caliber ammunition. The rebels surrounded a group of captured regime soldiers. The captives, some with bandages on their heads, each stood up and gave their rank and name.

The authenticity of such videos cannot be confirmed independently because Syria imposes tight restrictions on foreign journalists.

In another international entanglement, Assad ally Iran appealed to rebel backers Turkey and Qatar to help release 48 Iranians purportedly being held by Syrian rebels since August.

In amateur video posted late Thursday, rebels threatened to kill the captives by the end of the weekend unless the regime halts military operations against the opposition.

Iran's Foreign Minister Ali Akbar Salehi spoke by phone Saturday with the Qatari prime minister and the Turkish foreign minister and received assurances they would try to help, Iran's state news agency IRNA reported.

Iran says those abducted were pilgrims visiting a Shiite shrine in Damascus. The Syrian opposition claims they are members of Iran's Revolutionary Guards helping the Syrian regime.