Kyiv has urged Moscow to pull back its troops from Ukraine’s border and continue dialogue with the west if it is “serious” about de-escalating tensions that have soared amid fears of a Russian invasion. “If Russian officials are serious when they say they don’t want a new war, Russia must continue diplomatic engagement and pull back military forces it amassed along Ukraine’s borders and in the temporarily occupied territories of Ukraine,” Kyiv’s foreign minister, Dmytro Kuleba, tweeted on Sunday. Meanwhile, the head of Nato said Europe needed to diversify its energy supplies, as Britain warned it was “highly likely” that Russia, a major natural gas supplier, was looking to invade Ukraine. Russia has massed 120,000 troops near its neighbour and demanded the western defence alliance pull back troops and weapons from eastern Europe and bar Ukraine, a former Soviet state, from joining Nato. US officials said on Saturday Russia’s military buildup had been expanded to include supplies to treat casualties of any conflict. Across the border in Ukraine, locals trained as army reservists as the government scrambled to prepare. Moscow denies it plans to invade but said on Sunday it would ask Nato to clarify whether it intended to implement key security commitments after earlier saying the alliance’s response to its demands did not go far enough. “If they do not intend to do so, then they should explain why,” Russia’s foreign minister, Sergei Lavrov, said on state television. “This will be a key question in determining our future proposals.” The US has said it is waiting to hear back from Russia. It says Nato will not withdraw from eastern Europe or bar Ukraine from possible future entry but it is prepared to discuss topics such as arms control and confidence-building measures. US senators are very close to agreeing on sanctions legislation, the two leading senators working on the bill said on Sunday. Measures include targeting the most significant Russian banks and Russian sovereign debt as well as offering more lethal assistance to Ukraine. Some of the sanctions in the bill could be imposed before any invasion because of what Russia had already done, said one of the senators, Bob Menendez, citing cyber-attacks on Ukraine, “false flag” operations and efforts to undermine the Ukrainian government internally. Washington has spent weeks trying to build agreement with its European partners on having a strong sanctions package in place if Russia were to attack Ukraine. But the issue is divisive, with Germany urging “prudence”. “We are concerned about the energy situation in Europe because it demonstrates the vulnerability of being too dependent on one supplier of natural gas and that’s the reason why Nato allies agree that we need to work and focus on diversification of supplies,” Jens Stoltenberg, the Nato secretary general, said. Britain said on Sunday it would expand the scope of its own possible sanctions in legislation this week to deter the Russian president, Vladimir Putin. “We think it’s highly likely that he is looking to invade Ukraine. That is why we’re doing all we can through deterrence and diplomacy, to urge him to desist,” the foreign secretary, Liz Truss, told the BBC. Truss, who is due to visit both Ukraine and Russia in the next two weeks, told Sky News the legislation would enable Britain to hit a much wider variety of targets “so there can be nobody who thinks that they will be immune to those sanctions”. Asked if the new powers could include the ability to seize property in London, Truss said: “Nothing is off the table.“ The Center for American Progress, a US thinktank, has said Britain would face a challenge uprooting wealthy Russians with Kremlin links from London given close ties “between Russian money and the United Kingdom’s ruling Conservative party, the press, and its real estate and financial industry”. Asked about this, Truss said: “There’s a real threat here to freedom and democracy in Europe. And that is more important than short-term economic gains, both for the United Kingdom but also for our European allies.” The Biden administration planned to spare everyday Russians from the brunt of US export controls if Russia invades Ukraine, and focus on targeting industrial sectors, a White House official said on Saturday. A top commerce official said earlier that “key people” would face “massive sanctions”. The British prime minister, Boris Johnson, is expected to speak to Putin by phone next week. Stepping up diplomatic efforts after facing criticism for not doing enough, he said he had ordered the military to prepare to help strengthen Europe’s borders. Stoltenberg said Nato had no plans to deploy combat troops to non-Nato member Ukraine in the event of a Russian invasion, adding: “We are focusing on providing support.” Johnson said on Sunday the picture on Ukraine’s border with Russia was “increasingly concerning”. “I continue to urge Russia to engage in negotiations and avoid a reckless and catastrophic invasion,” the prime minister said on Twitter.