The US has called for a war crimes investigation into Russia's actions in Syria - saying Moscow owes the world an explanation.

Secretary of State John Kerry said Russian and Syrian strikes on hospitals "beg for an appropriate investigation of war crimes".

He said such attacks are "way beyond" accidental at this point, and accused Moscow and Damascus of undertaking a "targeted strategy ... to terrorise civilians".

Mr Kerry, who was speaking in Washington alongside his French counterpart Jean-Marc Ayrault, said Syrian forces hit another hospital overnight, killing 20 people and wounding 100.

Mr Ayrault spoke of a new French effort for a ceasefire in Syria. But it is unclear what advantages this plan would have over the US-Russian led process that collapsed last month.

The US-Russia agreement would have created a new counter-terrorism alliance in Syria, had fighting stopped for a week and aid deliveries been permitted to reach civilians. Neither condition was met.

The truce broke down completely when Syria and Russia renewed their military offensive in Aleppo.

Mr Kerry ended bilateral talks with Moscow on the military partnership earlier this week.

The latest remarks came after the lower house in Russia's parliament ratified a treaty with Syria that allows the Russian military to stay indefinitely in the Middle Eastern country.

The Kremlin-controlled State Duma voted unanimously to ratify the agreement, which formalises Moscow's military presence at the Hemeimeem air base in Latakia.

It is a show of support for the regime of Syrian President Bashar al Assad, and allows Russia to use the base free of charge and for as long as it needs.

Russia began its air campaign in Syria a year ago, reversing the tide of the nation's five-year civil war and helping Assad's forces win back key ground.

Moscow says its aim is to help the Syrian army fight terrorism.