* Rights group had been tracking the ship
* Group says Russian ship turned transponder off (Adds details, refusal to disclose cargo manifests, Iran)
By Louis Charbonneau
UNITED NATIONS, May 31 (Reuters) - 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 al-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 U.N. mission said he would look into the issue.
In a letter to the U.N. 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.
U.S. 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.
"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.
RUSSIA, CHINA OPPOSE ARMS EMBARGO
Hameed said that Human Rights First had been tracking the ship from May 23-30 and discovered "a window of time on May 26 when the ship's transponder appears to have shut off." A Western diplomat said turning off a transponder would be a violation of International Maritime Organization regulations.
Hameed also said that Syrian and Russian port authorities have refused to disclose the cargo manifests
Al Arabiya television first reported about the arms shipment last week. One Western diplomat who confirmed the report at the time said the ship is owned by a Maltese firm, which itself is owned by a Cypriot company that is owned by Russian firm.
Diplomats said the Russian firm might have been acting on behalf of state arms exporter Rosoboronexport, though that was not clear. Rosoboronexport could not immediately be reached for comment. Last week the company declined to comment on the ship.
The Russian Foreign Ministry said last week that it did not have information that a ship was headed to Tartus with weapons and declined to comment further.
Syria is one of Russia's top weapons customers. The United States and European Union have suggested the U.N. Security Council should impose an arms embargo and other U.N. sanctions on Syria for its assault on an opposition determined to oust Assad.
U.S. and European security officials say Iran is also providing a broad array of assistance to Assad to help him suppress anti-government protests, from high-tech surveillance technology to guns and ammunition.
But Russia, with the support of fellow veto power China, has prevented the council from imposing any U.N. sanctions on Syria and has refused to halt arms sales to Damascus.
The Russian Foreign Ministry said it did not have information that a ship was headed to Tartus with weapons and declined to comment further.
Russia has defended its weapons deliveries to Syria in the face of Western criticism, saying government forces need to defend themselves against rebels receiving arms from abroad. Damascus says Qatar, Saudi Arabia and Libya are among the countries helping the rebels.
Russian U.N. Ambassador Vitaly Churkin on Wednesday rejected criticism of Moscow's arms sales to Damascus.
"The weapons we may have provided to Syria under various contracts, which were concluded a long time ago, are fully in line with international law and do not contribute to the current armed violence in Syria," he said. (Additional reporting by Mark Hosenball in Washington and Michelle Nichols in New York; Editing by Vicki Allen; Editing by Anthony Boadle and Vicki Allen)