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US President Donald Trump says his country's relationship with Russia is at a "dangerous low", amid a feud with Congress over new sanctions imposed against Moscow.

Mr Trump approved the measure on Wednesday, despite calling it "flawed".

Russia said the new sanctions were tantamount to declaring a "full-scale trade war".

The law aims to punish Russia for its alleged meddling in the 2016 US elections and its actions in Ukraine.

Mr Trump had opposed the bill, which also contains measures against Iran and North Korea, as it constrains his ability to ease the sanctions without the consent of Congress.

The final line of his signing statement argued Congress was making a mistake, saying "as president, I can make far better deals with foreign countries than Congress".

'You can thank Congress'

In a fresh outburst on Thursday, he made clear who he thought was responsible for the current state of US-Russia ties.

"Our relationship with Russia is at an all-time & very dangerous low," he wrote on Twitter.

"You can thank Congress, the same people that can't even give us HCare!" he added, in reference to his inability to repeal and replace his predecessor Barack Obama's flagship health reforms.

But the Republican senator for Arkansas, Tom Cotton, told MSNBC that "our relationship with Russia is at a very low point but it's [Russian President] Vladimir Putin's fault".

Analysis: Trump still bitter

Anthony Zurcher, North America reporter

When presented with a choice of whom to blame for the "dangerous low" state of US-Russia relations, Donald Trump pointed his finger at Congress.

The president is still clearly bitter that the Senate rejected the healthcare reform he backed. In fact, the most significant legislation presented for Mr Trump's signature so far has been the Russia sanctions bill he reluctantly signed on Wednesday.

That was not part of the president's agenda, but he still has hopes for advancing his own priorities through Congress - particularly tax reform and changes to the US immigration system.

Taking swipes at a co-equal branch of the US government, however, isn't going to make things any easier for him.

Never mind that the relationship between the US and the Soviet Union was pretty darn for 45 years of "cold war"- including several moments when the two nations appeared on the brink of war.

Or that the president again seems to be downplaying the conclusion by the US intelligence community that Russia tried to influence the US 2016 presidential election.

If newly installed chief of staff John Kelly is bringing new discipline to this White House, that doesn't seem to extend to the presidential twitter account.

Russia has also reacted angrily to the sanctions package. Prime Minister Dmitry Medvedev said it "ends hopes for improving our relations with the new US administration".

In a Facebook post, Mr Medvedev said the measures showed Mr Trump's "total weakness" and that he had been humiliated by Congress.

A Kremlin spokesman, Dmitry Peskov, would not elaborate on Mr Medvedev's comments but said "Russia will defend and protect its interests".

Media playback is unsupported on your device Media caption What does body language tell us about the Trump-Putin G20 meeting?

Moscow had already retaliated last week by ordering the US to cut its diplomatic mission in Russia by 755 people.

The legislation limits the amount of money Americans can invest in Russian energy projects, and makes it more difficult for US companies to do business with Russia.

Trump and Russia

While running for president, Donald Trump heaped praise on Russian President Vladimir Putin, saying he wanted to see better relations between their two countries

But Mr Trump's time in office has been dogged by allegations Russia meddled in the election to help him win. Several investigations are under way to determine if his campaign colluded

Russia denies interfering and Mr Trump says there was no collusion, calling it a "witch hunt"

Claims of improper ties, and the emergence of undisclosed meetings with Russians, have led to the downfall of senior Trump officials and members of his close family are under scrutiny

Other factors have complicated ties too, such as the US raids on forces loyal to Syrian President Bashar al-Assad, Russia's ally

Russia: The 'cloud' over the White House