MPs have attacked UK ministers over their refusal to criticise Donald Trump’s attack on Venezuela, as Keir Starmer attempts to walk a fine line between defending international law and keeping the US president on side.

Labour, Conservative and Liberal Democrat MPs rounded on the foreign secretary, Yvette Cooper, on Monday night as frustration grew over the government’s cautious response to the US capture of the Venezuelan president, Nicolás Maduro.

Both Cooper and the prime minister have been reluctant to criticise Trump’s actions or to give an opinion on their legality, prompting concern among critics that the UK’s response will embolden China and Russia.

Speaking in the Commons on Monday evening, Emily Thornberry, the Labour chair of the foreign affairs select committee, said: “If a large and powerful country abducts the leader of another, however abhorrent that leader is, and then tries to intimidate the smaller country – in order it says itself to gain access to its resources – doesn’t the foreign secretary agree that this should be called out, not just by Britain, but by our western allies?”

She added: “We should be calling it out for what it is: a breach of international law.”

Those sentiments were echoed by colleagues from the left of the Labour party, but also from many on the Liberal Democrat and even the Conservative benches.

Richard Burgon, a Labour MP, said: “It was the prime minister that decided to disregard the United Nations charter when it came to Trump’s bombing, killing, and [the] kidnapping of a head of state. I think it speaks volumes that the prime minister has chosen not to come to this house to explain his decision.”

Ed Davey, the Liberal Democrat leader, said: “Maduro is a brutal, illegitimate dictator, but that does not give President Trump a free pass for illegal action.”

The former Conservative foreign secretary, Jeremy Hunt, warned that the entire future of Nato could be at risk if western allies did not face down American threats to launch an attack on Greenland next.

“If the new US approach extends to the annexation of Greenland – the sovereign territory of a Nato member – it could mean the end of the alliance with disastrous consequences.

“So how much planning is going on at the Foreign Office to actually make sure such a disaster doesn’t happen? We understand the discussions between allies have to be in private, but what it looks like from the outside is that Europe is weak and divided.”

The MPs’ frustrations were voiced after a tense day of international diplomacy in which Cooper spoke to both her US counterpart, Marco Rubio, and María Machado, the prominent Venezuelan opposition politician.

Cooper told the Commons she had stressed the importance of international law in her conversation with Rubio, but would not say whether she saw the US action as illegal, saying it was for Washington to set out its justification.

Starmer spent much of the day preparing for a meeting of the “coalition of the willing” – the group of countries that are prepared to contribute to the long-term security of Ukraine. That group has been keen to keep Washington on side as it prepares for the possibility of a peace deal between Moscow and Kyiv.

The prime minister said on Monday he wanted to “establish the facts” before passing judgment on whether Trump broke international law with the attack on Caracas.

He did however mount a robust defence of Denmark after Trump reiterated suggestions the US could annexe Greenland.

“Denmark is a close ally in Europe, it is a Nato ally,” he said. “It’s very important the future of Greenland is for the kingdom of Denmark and for Greenland, and only for Greenland and the kingdom of Denmark.”

Those comments came hours after the Home Office minister, Mike Tapp, attracted criticism for refusing to say that the UK would defend Denmark over the future of Greenland.

In broadcast interviews earlier on Monday, Tapp refused to directly criticise the idea of a US takeover of Greenland, telling Sky News: “Diplomacy is delicate, which means we’re not here to give a running commentary in the news.”