US envoy Susan Rice on Thursday condemned "reprehensible" Russian arms deliveries to Syria as she stepped up US calls for increased international pressure on President Bashar al-Assad.

The US ambassador to the United Nations also accused the Damascus government of stating a "blatant lie" by denying it was involved in a massacre in which 108 people were killed.

A Russian ship carrying arms arrived in the Syrian port of Tartus last weekend, according to media reports and rights groups.

Russia is Syria's last main international ally and has defended Assad in UN Security Council debates on the uprising against him. Russia's main Mediterranean naval base in Tartus.

"With respect to the reported docking of a ship carrying Russian arms, this is obviously of the utmost concern given that the Syrian government continues to use deadly force against civilians," Rice told reporters at UN headquarters.

"It is not technically, obviously, a violation of international law since there is not an arms embargo, but it is reprehensible that arms would continue to flow to a regime that is using such horrific and disproportionate force against its own people," Rice said.