The White House has made a late push to clinch ratification of a new nuclear arms reduction treaty with Russia, as a final vote on the agreement – one of Barack Obama’s top foreign policy priorities – looms this week.

Amid continued uncertainty about the ability of the administration to muster the two-thirds majority required in the Senate, Mr Obama sought to reassure recalcitrant Republicans that the new Start treaty “places no limitations” on the roll-out of missile defence systems in Europe.

In a letter to senators on Sunday, Mr Obama added that he would “take every action available to me to support the deployment of all four phases” of missile defence. The preamble of the new nuclear arms pact with Russia cites an “interrelationship” between offensive and defensive strategic weapons, and opponents argue this could block the US from moving forward with its European missile defence programme, making it a key obstacle to ratification.