UNITED NATIONS - The United States described Moscow's latest arms sale to Syria as "reprehensible" after a rights group reported on Thursday that a Russian cargo ship heavily laden with weapons arrived in Syria last weekend.

Western officials confirmed information from Human Rights First that the ship arrived on Saturday, bringing to the Syrian port of Tartus a cache of heavy weapons for Syrian President Bashar Assad, who for 14 months has used his security forces to attack an increasingly militarized opposition.

"Today's updated shipping databases show that the Professor Katsman did in fact dock in the port of Tartus on May 26, 2012 before heading to Piraeus, Greece," Sadia Hameed of Human Rights First told Reuters.

One Western diplomat told Reuters the shipment included heavy weapons, though it was not immediately clear what kind of heavy arms.

A spokesman for Russia's UN mission said he would look into the issue.

In a letter to the UN Security Council last week, Secretary-General Ban Ki-moon said he had seen reports of countries supplying arms to the government and rebels. He urged states not to arm either side in the Syrian conflict.

US Ambassador to the United Nations Susan Rice had tough words for Moscow, a staunch ally of the Assad government and one of its main weapons suppliers.

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"This is obviously of the utmost concern given that the Syrian government continues to use deadly force against civilians," she told reporters.

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

Last week more than 100 people were killed in a massacre in Houla, Syria, which the United Nations said appears to have been the work of the Syrian army and allied militia. Damascus has accused the rebels of committing the atrocity.

The top UN human rights body is set to call on Friday for a full UN inquiry into the Houla massacre after putting initial blame on government bombardment and gunmen loyal to Assad, diplomats said.

The 47-state forum in Geneva holds an emergency session on Friday - its fourth on Syria in a year - following last week's slaughter of at least 108 people, nearly half of them children.

A draft resolution, circulated late on Thursday at the UN Human Rights Council, condemns the "killings confirmed by UN observers" in attacks that involved "the wanton killings of civilians by shooting at close range and by severe physical abuse by pro-regime elements and a series of government artillery and tank shellings of a residential neighborhood".

The Council, which has repeatedly condemned Syria for its crackdown, is likely to adopt the resolution by a wide margin following international outrage at the killings, even if countries including China, Cuba and Russia may vote against it as in the past, Arab and Western diplomats said.

But the text, co-sponsored by the United States, Qatar and Turkey, was not strong enough to win the backing of the European Union, diplomats said.

Negotiations continue on Friday ahead of the special session, scheduled to start at 0900 GMT.

"The general feeling is that the text is weak. That is particularly why the EU is not on board," an Arab diplomat told Reuters, speaking on condition of anonymity.

The massacre, a clear breach of an April 12 ceasefire deal brokered by international mediator Kofi Annan, has prompted leading Western nations to expel Syria's envoys from their capitals.

The office of U.N. High Commissioner for Human Rights Navi Pillay said on Tuesday that most victims were civilians and entire families were shot in their homes. Witnesses told U.N. investigators that most died in summary executions carried out by "shabbiha" militiamen loyal to Assad.

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