The broad outlines of the US-Iran deal under which five US citizens detained by Iran would be allowed to leave in exchange for the transfer of the funds and the release of five Iranians held in the US were made public.

The swap of Brittney Griner for convicted Russian arms dealer Viktor Bout raised questions as to why the US side had failed to secure the simultaneous release of Paul Whelan, a former Marine accused by Moscow of spying.

Viktor Bout, who was accused of arming rebels in some of the world's bloodiest conflicts, was detained in a US sting operation in Thailand in 2008, extradited to the US and sentenced in 2012 to 25 years behind bars.

The US has already made what Secretary of State Antony Blinken calls a 'substantial offer' to secure the release of Americans detained in Russia, including basketball star Brittney Griner and former Marine Paul Whelan.

The Ukrainian defence ministry says most of the exchanged soldiers had been seriously wounded by bullets or shell fragments, while others were suffering from burns and fractures.