Updated

U.S. Secretary of State Hillary Rodham Clinton sparked controversy this week when she accused Russia of escalating the conflict in Syria by sending it attack helicopters. Russian Foreign Minister Sergey Lavrov rejected the claim, saying that Russia is only shipping air defense systems under previously signed contracts.

The State Department acknowledged Thursday that the helicopters Clinton referred to were being returned to Syria after refurbishment and are not new tools against Syrian opposition groups. Nonetheless, Russia has shipped billions of dollars' worth of missiles, tanks, artillery and other military gear to Syria over more than four decades.

Here is a brief look at some weapons systems Russia has recently shipped to Syria or pledged to deliver, according to official statements and Russian media reports:

COMBAT HELICOPTERS

Syria has received dozens of Soviet-built Mi-8 and Mi-17 transport helicopters and Mi-24 helicopter gunships since the Cold War with the last deliveries taking place in the 1990s. Some of them require major repairs that can only be done by Russian repair plants.

State Department spokeswoman Victoria Nuland said the three refurbished helicopters were on their way to Syria after being out of commission for at least six months. She said that although they are not new tools, the U.S. is nevertheless concerned that the helicopters will be used by President Bashar Assad's regime to kill civilians.

PANTSYR-S1 AIR DEFENSE SYSTEM:

The truck-mounted short- and medium-range system combines air defense missiles and anti-aircraft artillery with sophisticated radar to hit aerial targets with deadly precision at ranges of up to 20 kilometers (more than 12 miles) and an altitude of 15 kilometers (nearly 50,000 feet). It has further strengthened Syria's air defense system, which has been developed with Moscow's help since Cold War times.

Igor Sevastyanov, a deputy head of the Rosoboronexport state arms trader, said Wednesday that the Pantsyr contract is still being implemented. Sevastyanov didn't offer specifics, but Russian media reports have said that the contract envisioned the delivery of 36 such truck-mounted units which combine guns and missiles together with a radar.

BUK-M2 AIR DEFENSE SYSTEM

The medium-range missile system is capable of hitting enemy aircraft and cruise missiles at ranges of up to 50 kilometers (31 miles) and an altitude of up to 25 kilometers (82,000 feet). It is a sophisticated weapon that is capable of inflicting heavy losses to enemy aircraft if Syria comes under attack.

BASTION ANTI-SHIP MISSILE SYSTEM

Armed with supersonic Yakhont cruise missiles that have a range of up to 300 kilometers (162 nautical miles), it provides a strong deterrent against an attack from the sea. Russian Defense Minister Anatoly Serdyukov said last fall that Moscow would fully honor the Bastion contract. Russian media reports said that Russia already has fulfilled the Bastion deal, which was worth $300 million, and included the delivery of more than 70 Yakhont missiles.

YAK-130 COMBAT JETS

Russian media reports said early this year that Syria had ordered a batch of 36 Yak-130 combat jets worth $550 million. Officials wouldn't confirm or deny the deal, which would significantly bolster the Syrian air force capability. The Yak-130 is a combat training jet that can also carry modern weapons for ground attack missions. Analysts say early deliveries are unlikely as the aircraft's pace of production is very slow.

RUSSIA'S STANCE

The Kremlin has insisted that the continuing Russian arms sales don't violate any international agreements and scoffed at Western demands to halt the trade. Underlining Moscow's defiance, a Russian ship carrying a load of weapons arrived In Syria just a few weeks ago amid international anger over Assad's refusal to honor a U.N.-sponsored peace plan.

The new Russian weapons supplies add to Syria's massive arsenal of hundreds of Soviet-built combat jets, attack helicopters and missiles and thousands of tanks, other armored vehicles and artillery systems. Russia said it also has military advisers in Syria training the Syrians to use the Russian weapons, and has helped repair and maintain Syrian weapons.