U.S. Assistant Secretary of State Jeff Feltman will request Russia to comment on the recently signed contract to sell combat trainer jets to Syria during his visit to Moscow, the U.S. Department of State said, according to RIA Novosti.

Earlier, the Kommersant daily quoted a source close to Russia's state arms exporter Rosoboronexport as saying Russia and Syria had signed a $550-million contract on the delivery of 36 Yakovlev Yak-130 Mitten combat trainer in late December.

“Well, we've seen the same press reports that you've seen. Obviously, if it's accurate, it would be quite concerning,” State Department spokeswoman Victoria Nuland said. “And issue number one on his agenda there is Syria… So I'm sure that Jeff will be raising these concerns as well.”

Earlier this month, the United States expressed concerns over weapons deliveries to Damascus following media reports about a Russian ship loaded with arms docking in the Syrian port of Tartus.

Nuland said Russia is yet to provide any official information on the ship and its cargo.