Washington - US President Donald Trump claimed relations with Russia had hit an all-time and "very dangerous" low Thursday, putting the blame on Congress after he reluctantly approved sanctions against Moscow.After Moscow called the sanctions a declaration of "economic war" that had exposed the US president's weakness, an angry Trump lashed out at his own lawmakers who had overwhelmingly approved the measures."Our relationship with Russia is at an all-time & very dangerous low," Trump wrote on Twitter."You can thank Congress, the same people that can't even give us HCare!" he added in reference to a recent defeat in the Senate on his health care reform plans.Senator John McCain, a noted Russia hawk, hit back on Twitter, suggesting Trump had misdiagnosed the source of the problem."Our relationship w/ Russia is at dangerous low. You can thank Putin for attacking our democracy, invading neighbors & threatening our allies."Trump's outburst came a day after he grudgingly signed off on the sanctions, calling the legislation "significantly flawed" and adding that some of its provisions were unconstitutional.Trump's presidency has been overshadowed by allegations that his campaign team colluded with Moscow during last year's US presidential campaign in which he defeated Hillary Clinton.After meeting his Russian counterpart at a G20 summit in Germany last month, Trump said he wanted to work more closely with Moscow on areas such as the conflict in Syria.But the legislation - which also includes measures against North Korea and Iran greatly limits his room for maneuver and underlines the lack of trust from lawmakers, even though his own Republican Party controls both houses of Congress.The sanctions target the Russian energy sector in particular, giving Washington the ability to sanction companies involved in developing Russian pipelines.It also places curbs on some Russian weapons exporters and constrains Trump's ability to waive the penalties.'Economic war'Moscow, which announced a series of retaliatory measures over the weekend, has painted the bill's passage in the Senate as a humiliation for Trump.Prime Minister Dmitri Medvedev took to Facebook late Wednesday to say Trump's signing of the bill "ends hopes for improving our relations with the new US administration.""It is a declaration of a full-fledged economic war on Russia," Medvedev wrote."The Trump administration has shown its total weakness by handing over executive power to Congress in the most humiliating way."Moscow has already ordered the US to slash diplomatic staff in Russia by 755 personnel in response. The Kremlin said Trump's formal approval did not "change anything" and no further retaliation was planned.The head of Russia's largest oil firm Rosneft - which has already been targeted by earlier US sanctions imposed over the Ukraine crisis - promised the company would try to avoid more pain. Sanctions 'backfiring' "We will seek to work in such a way as to minimize the impact of the sanctions," Igor Sechin told Russian news agencies, adding that sanctions had "started backfiring" to damage American interests. Iran reacted angrily, saying the new sanctions against it "violated" its 2015 nuclear deal with world powers, and warned it would respond "appropriately."The sanctions seek to penalise the Kremlin for allegedly meddling in the 2016 US presidential election and for Russia's annexation of Crimea.Washington has already slapped several rounds of sanctions on Moscow over Ukraine since 2014, and in December then leader Barack Obama turfed 35 Russian diplomats out due to accusations of election interference.Trump had received the legislation on Friday, but waited until Wednesday to sign it.The delay had raised speculation that Trump might veto or try to somehow shelve the sanctions, which were approved in a 98-2 Senate vote.By signing it, he avoided the humiliating prospect of Congress overriding his veto.US Secretary of State Rex Tillerson said on Wednesday that he would meet with his Russian counterpart Foreign Minister Sergei Lavrov over the weekend, but warned US-Russia ties could still get worse.A special prosecutor is investigating whether Trump advisers colluded with what US intelligence has concluded was an attempt by Russia to covertly support the real estate mogul's 2016 campaign.The US president, who often called for warmer ties with Moscow during the White House race, has furiously denied the charge.