Moscow: Russian President Vladimir Putin, condemned by NATO for annexing Crimea, is now defying the US in Syria by sending more and deadlier arms to help Bashar al-Assad score a string of advances against insurgents, military experts say.

Dr Assad's army, seeking to end a three-year civil war that has killed 150,000 people and displaced 9 million, started using longer-range Russian Smerch and Uragan rockets for the first time in February, according to Jane's Defence Weekly and Stratfor, a US geopolitical research company.

A Syrian woman reacts following an air strike by Syrian government forces in the al-Sakhour district of the northern city of Aleppo on April 2. Credit:AFP

"Russia is now doing everything to ensure that Assad wins convincingly," Alexei Malashenko, a Middle East analyst at the Moscow Carnegie Center, said by phone. "If Russia can show it's capable of carrying out its own foreign policy, regardless of America's wishes, it will be a major achievement for Putin."

Mr Putin, who last year averted US airstrikes on Syria by brokering a chemical weapons accord, is seeking to prolong the rule of his closest Arab ally, ignoring US and European Union calls for Dr Assad to step down.