The separate agreements are the first formal commitments by the warring sides since the inauguration of Donald Trump, who is pushing for an end to the war and a rapid rapprochement with Moscow that has alarmed Kyiv and European countries.
The US agreement with Russia goes further than the agreement with Ukraine, with Washington committing to help seek the lifting of international sanctions on Russian agriculture and fertiliser exports, long a persistent Russian demand.
The Kremlin said the Black Sea agreements would not come into effect unless links between some Russian banks and the international financial system were restored.
Ukrainian President Volodymyr Zelenskiy said this was untrue, and there was no requirement of sanctions relief for the deals to come into force.
"Unfortunately, even now, even today, on the very day of negotiations, we see how the Russians have already begun to manipulate," Zelenskiy said in his nightly video address. "They are already trying to distort agreements and, in fact, deceive both our intermediaries and the entire world."
Kyiv and Moscow both said they would rely on Washington to enforce the deals, while expressing scepticism that the other side would abide by them.
Russian Foreign Minister Sergei Lavrov said: "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 Zelenskiy and his team to do one thing and not the other."
Zelenskiy said the truce agreements would take effect immediately, and that if Russia violated them he would ask Trump to impose additional sanctions on Moscow and provide more weapons for Ukraine.
"We have no faith in the Russians, but we will be constructive," he said.
The deals were reached after parallel talks in Saudi Arabia that followed separate phone calls last week between Trump and the two presidents, Zelenskiy and Vladimir Putin.
Putin rejected Trump's proposal for a full ceasefire lasting 30 days, which Ukraine had previously endorsed.
But Washington has nevertheless softened its rhetoric towards Russia in recent days, with Trump envoy Steve Witkoff saying he did not "regard Putin as a bad guy", alarming European officials who consider the Russian leader a dangerous enemy.
Ukrainian Defence Minister Rustem Umerov said Kyiv would regard any movement of Russian military vessels outside the eastern part of the Black Sea as a violation and a threat, in which case Ukraine would have the full right to self-defence.
PAUSE ON ATTACKS ON ENERGY FACILITIES
Russia has attacked Ukraine's power grid with missiles and drones throughout the war, arguing that civil energy infrastructure is a legitimate target because it helps Ukraine's warfighting capability.
More recently, Ukraine has been launching long-range strikes on Russian oil and gas targets, which it says provide fuel for Russia's troops and income to fund its war effort.
The Kremlin said the pause in attacks on energy would last for 30 days from March 18, when Putin first discussed it with Trump. Ukraine had said last week that it would accept such a pause only after a formal agreement.
The agreement on a truce at sea addresses an issue that was critical early in the war, when Russia imposed a de facto naval blockade on Ukraine, one of the world's biggest grain exporters, worsening a global food crisis.
More recently, maritime battles have been only a comparatively small part of the war since Russia withdrew its naval forces from the eastern Black Sea after a number of successful Ukrainian attacks.
Kyiv has been able to reopen its ports and resume exports at around pre-war levels, despite the collapse of a previous UN-brokered Black Sea shipping agreement, but its ports have come under regular air attack. Zelenskiy said the agreement would bar such strikes.
Moscow said the agreement would require sanctions relief including restoring links between Russia's agricultural export bank and the SWIFT international payments system. That and other steps could require agreement from European countries.
Trump has been pressing Moscow and Kyiv to bring a rapid end to the war, a goal he promised to achieve when he ran for president last year. At the same time he is pursuing a rapid rapprochement with Russia that Washington and Moscow say could lead to lucrative business opportunities.
Ukraine and its European allies fear Trump could strike a hasty deal with Putin that undermines their security and caves in to Russian demands, including for Kyiv to abandon its NATO ambitions and give up land claimed by Moscow.
