Senior Ukrainian officials visited Washington on Tuesday seeking U.S. support against Russia, as Kyiv showed its ability to fight on by setting off an explosive device under a bridge that has become a symbol of the Kremlin's claims on Ukrainian territory.

A day after talks in Istanbul that made little progress toward ending the war, now in its fourth year, Ukraine's SBU security service said it had hit a road and rail bridge that links Russia and Crimea below the water level with explosives. The extent of any damage was not clear.

The bridge is a flagship project for Russian President Vladimir Putin, built after he annexed Crimea from Ukraine in 2014, in a precursor to the latest conflict.

Kremlin accuses Ukraine of targeting civilians

Kyiv appears determined to show U.S. President Donald Trump that it can still fight on, despite the rising death toll and destruction in the deadliest conflict in Europe since the Second World War.

Ukrainian forces also targeted the southeastern Zaporizhzhia region and the Kherson region in the south with shelling, which damaged electricity substations, leaving at least 700,000 people without power, Russia-installed officials said.

WATCH | Ukraine says it hit Russia's Crimea bridge: 

Ukraine says it hit bridge connecting Crimea to Russia

Ukraine's security service says it hit the Kerch Strait Bridge, an important route linking Russia and the Crimean Peninsula. Reuters verified the location of the video released by the security service by matching structural elements of the bridge in the video to satellite and file images, but was not able to independently verify the date.

The fresh attacks followed drone strikes over the weekend on Russian military airfields, some of which housed long-range nuclear-capable bombers.

Ukraine's success in striking deep into Russia has prompted calls by some Russian military bloggers for a harsh response.

Russia's state investigative committee accused Ukraine on Tuesday of carrying out "acts of terrorism" by blowing up two railway bridges in Russia over the weekend.

The attacks were planned to target hundreds of civilians, the committee said on Telegram. It said seven people were killed and 113 injured, including children, when two trains crashed in Russia's Kursk and Bryansk regions as a result of the attacks.

WATCH l Breaking down the weekend's audacious Ukraine attacks inside Russia: 

Inside Ukraine’s secret deep strike against Russian bombers | About That

Ukrainian drones struck deep into enemy territory in Russia, with officials claiming the attack destroyed over 40 Russian warplanes. Andrew Chang explains how Ukraine is believed to have pulled off what it describes as its longest-range attack against Russia, and how it says it was able to do so undetected. Images provided by Getty Images, The Canadian Press and Reuters.

Ukraine officials look for support, Russian sanctions

Meanwhile, a Russian artillery strike in Sumy on Tuesday killed three people and wounded 25, local officials said. The northeastern city is home to more than 255,000 people and located just 30 kilometres from the Russian border.

"Eight of the wounded are in serious condition, and three of them are children," Ukraine's Health Ministry said in a statement.

Ukrainian President Volodymyr Zelenskyy said at least one rocket fired from a multiple rocket launcher had failed to detonate and lodged itself in an apartment building.

At Monday's talks in Istanbul, Russia told Ukraine it would agree to end the war only if Kyiv gives up big new chunks of territory and accepts limits on the size of its army. Ukraine rejects the Russian conditions as tantamount to surrender.

"The [peace] settlement theme is extremely complex; it consists of a large number of nuances," said Kremlin spokesperson Dmitry Peskov, adding "it would be wrong to expect any immediate solutions and breakthroughs here."

Andriy Yermak, chief of staff to Zelenskyy, was set to visit the White House along with Yulia Svyrydenko, Ukraine's deputy prime minister. Ukraine says Moscow is stalling the peace talks and Yermak signalled that he would press Ukrainian demands for tougher sanctions on Russia.

"We will actively promote issues that are important for Ukraine. Our agenda is rather comprehensive," Yermak said via the Telegram app after arriving in Washington.

"We plan to talk about defence support and the situation on the battlefield, strengthening sanctions against Russia."

Yermak said the officials would also discuss a deal that gives the U.S. preferential access to new Ukrainian mineral projects and sets up an investment fund that could be used for the reconstruction of Ukraine.