Ukrainian President Volodymyr Zelensky has said a truce with Russia covering the Black Sea and energy infrastructure is effective immediately and that he would seek more weapons and sanctions on Russia from US President Donald Trump if Moscow broke the deals.
The United States said earlier it had made separate agreements with Ukraine and Russia to ensure safe navigation in the Black Sea and to implement a ban on strikes against energy facilities in the two countries.
"If the Russians violate this, then I have a direct question for President Trump. If they violate, here is the evidence - we ask for sanctions, we ask for weapons, etc," Mr Zelensky told reporters at a news conference in Kyiv.
The Kremlin also confirmed that it has agreed to ensure safe navigation in the Black Sea, provided that Western sanctions are lifted against companies involved in food and fertiliser exports.
Russia and the United States have also agreed to develop measures to halt strikes on Russian and Ukrainian energy facilities for a period of 30 days that began on 18 March, it added.
The Kremlin also confirmed that the US agreed to help to restore Russia's access to the world market for agricultural and fertiliser exports, but outlined a number of conditions.
It said implementation of both parts of the agreement would require Western sanctions on Russia's Rosselkhozbank, which services agriculture firms, to be lifted, and the bank's access to the SWIFT international messaging system restored.
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Sanctions on Russian food and fertiliser exporters, insurance firms, servicing food and fertiliser shipments, restrictions on vessels and trade finance operations, would have to be lifted as well, it said.
The agreements, if implemented, would represent the clearest progress yet towards a wider ceasefire that the US sees a stepping stone towards peace talks to bring an end to Russia's three-year-old war in Ukraine.
Russia, however, said it could not trust Mr Zelensky and it could therefore only sign a Black Sea deal if the US issued an "order" to him to respect it.
"We will need clear guarantees. And given the sad experience of agreements with just Kyiv, the guarantees can only be the result of an order from Washington to Zelensky and his team to do one thing and not the other," Russian Foreign Minister Sergei Lavrov said in televised comments.
It was not immediately clear whether Russia's demand risked derailing the deal.
Mr Zelensky has previously said Russian President Vladimir Putin, who ordered the invasion of Ukraine in February 2022, is not to be trusted over peace moves.
Ukrainian Defence Minister Rustem Umerov said Ukraine had agreed to both a maritime ceasefire and a pause by Russia and Ukraine in attacks on each other's energy infrastructure.
However, he said Ukraine would regard any movement of Russian military vessels outside the eastern part of the Black Sea as a violation and a threat.
In this case, Ukraine would have the full right to exercise self defence, he said.
Seeking to fulfil a pledge by Mr Trump to end the war quickly, the US originally proposed a full 30-day ceasefire - to which Ukraine agreed in principle on 11 March - as a step towards peace talks.
But the Americans held separate talks in Saudi Arabia with Russia and Ukraine this week to discuss more limited ceasefires on energy and at sea, after Mr Putin responded to the wider truce plan with a long list of conditions and questions.
Mr Trump is pressing both sides to bring a swift end to the war, something he promised to achieve when he ran for president last year.
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At the same time, he is pursuing a rapid rapprochement with Russia that both sides claim could lead to lucrative business opportunities and cooperation across a wide range of areas, from minerals to sport and space exploration.
Ukraine and its European allies fear Mr Trump could strike a hasty deal with Mr Putin that undermines their security and caves in to Russian demands, including for Ukraine to abandon its NATO ambitions and give up the entirety of four regions claimed by Russia as its own. Ukraine has rejected that as tantamount to surrender.
Maritime truce
The talks in Saudi Arabia follow phone calls last week between Mr Trump and Mr Putin and Mr Zelensky respectively. Ukrainian officials met the US delegation in Saudi Arabia on Sunday.
Mr Trump, who has scaled back US diplomatic backing for Ukraine and shifted publicly to a stance far less critical of Russia than that of his predecessor Joe Biden, says he aims to bring a quick end to the war.
The White House said the initial aim of the Saudi talks was to secure a maritime truce in the Black Sea, to allow the free flow of shipping.
However, maritime battles have been a comparatively limited facet of the war since 2023, after Ukrainian attacks drove Russia to move its navy far from contested waters, making it possible for Ukraine to reopen ports and resume exports.
"This is primarily about the safety of navigation," Kremlin spokesman Dimitry Peskov said. He said a previous UN-backed agreement on Black Sea shipping had failed to deliver some of Moscow's demands.
A source briefed on planning for the Saudi talks said the US side was led by Andrew Peek, a senior director at the White House National Security Council, and Michael Anton, a senior State Department official.
Mr Trump had expressed broad satisfaction over the way talks progressed and had been complimentary about Mr Putin's engagement.
However, major European powers doubt whether Mr Putin is ready to make real concessions or will stick to what they see as his maximalist demands, which do not appear to have changed since he sent troops into Ukraine in 2022.
Mr Putin has said he is ready to discuss peace but that Ukraine must officially drop its NATO ambitions and withdraw its troops from the entirety of four Ukrainian regions that Russia has unilaterally annexed.
White House national security adviser Mike Waltz said that beyond a Black Sea ceasefire, the teams in Riyadh, would discuss the "line of control" between the two armies, which he described as "verification measures, peacekeeping, freezing the lines where they are".
