Russia has signed a deal to sell Syria nearly 40 fighter jet trainers for more than half a billion dollars, a Russian newspaper reported on Monday. The deal comes despite growing international criticism over Russian military trade with Syria, where more than 5,000 people have been killed as President Bashar Assad uses troops and tanks to try to crush the 10-month-old popular revolt.

The Russian daily Kommersant cited a source close to Russia's state arms export monopoly, Rosoboronexport, as saying that the sides had signed a contract after holding talks in December, and that Damascus was to pay $550 million for 36 Yak-130 aircraft.

A spokesman for Rosoboronexport refused to comment. A Russian defense industry source indicated the planes had not been built yet, saying assembly could start after Syria made a down payment, according to Kommersant.

"The creation of 36 new fighters for a foreign consumer is fully within the power of [manufacturing facilities]," the source was quoted as saying. "As soon as Syria sends Russia the down payment, the factory can get started on assembly."

The advanced training jets could be used for air strikes on ground targets and to train pilots on the country's more advanced fleet of Mig-29 jet fighters, which it ordered from Russia in 2007.

Moscow is one of Assad's few remaining allies, still serving top arms customer Syria while joining China in an October veto of a Western-crafted U.N. Security Council resolution that threatened an arms embargo.

Syria accounts for $700 million in arms sales in 2010, some 7 percent of Russia's total of $10 billion in arms deliveries abroad, according to the Russian defense think tank CAST.

CAST director Ruslan Pukhov said he had previously been aware of the deal, but said the contract might be annulled by the Syrian side if Damascus suspected that any international military action against Assad’s rule was in the works.

"It's an expensive order and while these jet trainers may be good for specific missions ... they are no substitute for a supersonic [fighter] jet," he said.

Russia has made strong gestures to reaffirm support for Assad recently, including sending its Navy flagship to a Russian base on the Mediterranean coast of Syria near Tartus.

Earlier this month a ship full of ammunition from Russia was detained in Cyprus. The ship was released the next day and sailed on to Tartus.

The U.S. said it had raised concerns about the ship with Russia, but Moscow has said it needs no justification for its defense trade with Syria without an internationally binding arms embargo in place.