Russia has sold advanced Yakhont antiship cruise missiles to Syria’s President Bashar Assad-led government, disregarding the U.S. and allies' request to stop arms sales to the regime, American officials said on Thursday.

The 300 Yakhont antiship missiles are an advanced version of the S-300 antiship cruise missiles that are fitted with an advanced radar system. The missile sale to the regime is considered an affront to the U.S. and its allies' attempts to bring peace to the region.

The powerful missile systems, nicknamed “ship killers,” can help Assad’s government counter any international forces that might send reinforcements or supplies to Syrian rebels by imposing a naval embargo in the region.

“It enables the regime to deter foreign forces looking to supply the opposition from the sea, or from undertaking a more active role if a no-fly zone or shipping embargo were to be declared at some point,” said Nick Brown, editor-in-chief of IHS Jane’s International Defense Review, the New York Times reported.  “It’s a real ship killer,” he added.

Russia, one of Syria’s strongest allies has been shipping advanced cruise missiles and other arms, citing a contractual obligation it has with the regime. According to the Times report, Syria placed orders for the S-300 antiship missiles to Russia in 2007, and Moscow supplied the first batch of missile systems in 2011.

Russian Foreign Minister Sergei Lavrov on Thursday claimed that the latest shipment of antiship missiles to Syria had no connection with Israel’s recent air strikes against Syria. He said that the consignment was part of an old contract signed before the air strikes were launched on Syria last year, the Guardian Express reported.

According to Lavrov, no new contracts have been signed between Russia and Syria. "We have already gone through with some of our previous commitments and we intend to complete them. We do not want to break any international law, but also do not want to hurt our good name as arms suppliers," Lavrov said, as reported by Israel’s YNet News.

Lavrov defended the arms sale, claiming that the missiles are defensive systems. In a statement that observers see as a veiled warning to Israel and the U.S., he said: “Those who do not plan aggressive actions against a sovereign state have nothing to worry about, because air defense methods -- and this is clear from the name -- are a purely defensive system required to repel air attacks.”

Nonetheless, his present statements contradict his own stance on the arms deal a week ago, political observers pointed out. Last week, in a statement, he said that Russia would not sell the advanced antiship cruise missiles to Syria.

The report of the Russian shipment comes as Washington and Moscow plan to convene an international conference to de-escalate the armed conflict in Syria. The conference scheduled to take place in June is expected to include representatives of the Syrian government and opposition forces, according to the Times report.

However, both the U.S. and the Russia differ on the scope of the proposed summit, as Moscow wants Iran also to be part of the summit, while the U.S. and Western countries are against any such move.

"One must not exclude a country like Iran from this process because of geopolitical preferences. It is a very important player. But there is no agreement on this yet," Lavrov said in an interview to a Lebanese television station.

Moscow’s continuing display of commitment to the Assad regime is a significant concern to the U.S. and international forces as such moves could embolden the Assad regime, which believes it can prevail over the opposition by using military force.  

“This weapons transfer is obviously disappointing and will set back efforts to promote the political transition that is in the best interests of the Syrian people and the region,” the Times reported, quoting Senator Bob Corker, the senior Republican on the Foreign Relations Committee of Tennessee.