Joe Biden issues ultimatum to Putin: US will strike back if Russia attacks. JOE BIDEN has warned the Russian President that now is the time to de-escalate tensions at the Ukrainian border and "refrain from military action". Speaking at the White House on Thursday, the US President also warned Vladimir Putin that a series of US sanctions on Russia could have been a lot tougher. But Mr Biden claimed he did not want to worsen already high tensions with Moscow. He said: “I was clear with President Putin we couldn't have gone further, but I chose not to do so. “I chose to be proportionate. The United States is not looking to kick off a cycle of escalation and conflict with Russia. We want a stable predictable relationship.” The US President also warned that if Russia were to retaliate, then American will push back stronger. He said: “I urged to respond appropriately, not to exceed it, because we can move as well.” “Where Russia seeks to violate the interests of the United States we will respond. We will always stand in defence of our country, our institutions, our people and our allies.” The US President has offered to hold a summit with Mr Putin in Europe this summer in a bid to resolve the escalating tensions. Russia’s actions on the Ukrainian border and violations of a cease-fire agreement in the Crimea have alarmed Washington and European leaders. Speaking at the White House press briefing, Mr Biden said: “I proposed that we meet in person this summer in Europe to address the range of issues facing both of our countries. The Biden administration had planned to send two US warships to the Black Sea but decided to cancel their deployment to avoid an unnecessary escalation of the situation. Russia has since closed part of the Kerch Strait in an attempt to block all foreign warships from reaching Ukraine, according to reports. The Kremlin had previously warned US warships to steer clear of Crimea “for their own good”. Ukraine has reportedly said the closure of the strait was an “act of war” and is said to be disappointed that foreign warships have turned away. Commercial ships will still be able to pass through the crucial Kerch Strait but the closure may leave the region vulnerable to Russian forces. In a televised briefing, Russia’s foreign ministry spokeswoman Maria Zakharova confirmed that US Ambassador to Russia John Sullivan had been summoned for a “difficult conversation”. Ms Zakharova said that a Russia response to Mr Biden’s sanctions was “unavoidable”. She added: “Such aggressive behaviour, without question, will receive a decisive push back. A response to the sanctions is unavoidable.