A senior US diplomat visiting Russia will press Moscow on a reported deal to sell Syria fighter jets, something Washington described as "quite concerning," the State Department said.

Jeffrey Feltman, the top US diplomat for the Middle East, was in Moscow on Monday and Syria was "issue number one on his agenda," US State Department spokeswoman Victoria Nuland told reporters in Washington.

In Moscow, the Kommersant business daily reported that Syria had signed a $550-million (425-million-euro) contract to purchase 36 Yak-130 advanced training/light attack planes from Russia.

"Every time we talk to the Russians about Syria, we make the point about how dangerous we think it is to be continuing to trade in weapons and encouraging them to do what they can to stop such trades," Nuland said.

"So with regards to these new reports, they came up today. We've sent them on for Ambassador Feltman to raise with the Russians," Nuland added.

"We've seen the same press reports that you've seen. Obviously, if it's accurate, it would be quite concerning."

On January 13, Nuland said Washington had raised concerns with Moscow over a Russian ship suspected of carrying munitions to Syria. A shipping expert said the ship arrived at the Syrian port of Tartus on January 11 or 12.

"We haven't really gotten much clarity on how that went down," Nuland said Monday about the Russian response.

Besides asking about the Russian weapons trade with Syria, she said Feltman would also press Moscow on its position on efforts at the UN to get Syrian President Bashar al-Assad to hand over power.

Germany is leading European demands for UN Security Council backing for an Arab League plan.

But the Security Council has been blocked for months over Syria, with Russia and China maintaining that any moves in the UN body against Assad would be the first steps toward regime change by force, as in Libya last year.

The UN says at least 5,400 people have died since protests against Assad erupted last March.