BEIRUT – U.S. Secretary of State Hillary Rodham Clinton accused Moscow on Thursday of contributing to a possible “catastrophic” civil war in Syria.

The Russians “keep telling me they don’t want to see a civil war, and I have been telling them their policy is going to help contribute to a civil war,” Clinton said. Russia’s actions in Syria are not a “stabilizing influence.”

The Russians “are, in effect, propping up the regime at a time when we should be working on a political transition,” Clinton said during a visit to Denmark.

At the United Nations, U.S. Ambassador Susan Rice described reports of Russian arms shipments to Syria as “reprehensible” after accounts surfaced that a weapons-laden Russian ship arrived in the port of Tartus last weekend.

There was no information from Moscow on the reported recent arms shipment, but in the past Russian officials have defended weapons sales to Syria as essential for Syria’s self defense.

Russia has been the most potent international ally of Syrian President Bashar Assad, twice vetoing United Nations Security Council resolutions that could have led to sanctions or other actions against the Syrian government for its crackdown on dissent. Washington and its allies in the West and the Arab world have called for Assad to step down. In February, Rice said she was “disgusted” by a Russian veto of a resolution condemning the Assad administration.

There has been no sign that Russia would change its stance on Syria after the massacre last week of more than 100 people, mostly children and women, in the Syrian township of Houla – an event that galvanized international outrage.

Although U.S. and U.N. officials have said the evidence in the case points to pro-government militiamen as the killers, Russia has responded that both government and rebel forces were probably involved.

On Thursday, Syrian authorities again denied responsibility for the Houla killings and blamed the deaths on anti-government “armed gangs.” Rice dismissed that as a “blatant lie.”

On Sunday, Russia did sign off on a nonbinding Security Council statement condemning Syria for its use of artillery and tanks in Houla. But that did not signal a shift in Moscow’s policy. Russia said it is not supporting Assad specifically but is backing the peace plan of Kofi Annan, the U.N. special envoy for Syria.

Moscow has lashed out at other governments for favoring “regime change” and aiding militant groups fighting the government in Syria.

“Russia’s position is well known, it is balanced, it is consistent and absolutely logical,” Dmitry Peskov, President Vladimir Putin’s spokesman, said on Thursday.