Senator John McCain turns down Ukraine advisor role
Senator John McCain (AFP Photo/Wakil Kohsar)

Republican US Senator John McCain has turned down a post as advisor to Ukraine's pro-Western President Petro Poroshenko, saying constitutional constraints prohibited him from accepting.

McCain, who has pressed Washington to send lethal weapons to war-torn Ukraine, rejected the offer to join an advisory council aimed at building global support for Ukraine citing his obligations as a US senator.

"I was honoured to be asked to join Ukraine's International Advisory Council on Reforms, a forum for discussing ways to ensure Ukraine's territorial integrity and security and support the country's democratic future in the face of Russian aggression," McCain said in a statement released on his official website in Washington Thursday.

"However, under provisions of the US Constitution concerning the interaction of Members of Congress with foreign governments, I am obligated to decline the invitation," the statement said.

"I will continue to do everything I can to stop Vladimir Putin's unabated aggression, and stand shoulder-to-shoulder with free Ukraine," McCain said.

Ukrainian forces have been battling pro-Russian separatists since April 2014 in eastern Ukraine in a bid to prevent the secession of the Russian-speaking regions of Donetsk and Lugansk.

The West and Kiev have accused the Kremlin of instigating the insurgency and supporting militants with weapons, funds and troops. Moscow has denied the claims.