Ukrainian forces are constantly working on fortifications, Ukrainian Prime Minister Denys Shmygal said on Monday, at a time when Russian troops are pushing in the east.
The statement comes amid some criticism of Ukrainian defence lines after the rapid advances of Russian forces following their capture of the eastern stronghold of Avdiivka.
"I am confident and know that the Ministry of Defence has an exhaustive plan to create comprehensive lines of fortification," Shmygal said at a press conference.
"Work is carried out on a daily basis, 24/7," he added.
Russia closing in
The Ukrainian government has allocated $500 million for fortifications this year, he added.
"This year, the government is even ready to proactively continue to allocate funds... for the construction of powerful fortifications -- not only on the first and second lines, but also on the third and, if necessary, the fourth," Shmygal said.
In late 2023, Ukrainian President Volodymyr Zelensky said that "mobilising resources for fortification" was a "priority".
Defence lines are made of trenches, bunkers and anti-tank traps -- including pyramid-shaped concrete blocks commonly known as "dragon's teeth".
Lines far from the front are usually built by civilian contractors.
Railway blast
Ukraine said it blew up a railway bridge in Russia's southwestern Samara region on Monday because it was being used to transport "military cargo".
The incident is the latest in a string of explosions targeting Russia's rail network, which Kyiv says Moscow uses to move troops and equipment for its invasion of Ukraine.
"A railway bridge over the Chapayevka River in Russia's Samara region was blown up. On 4 March 2024, at around 6:00 am (2am GMT), the bridge was damaged by blowing up its support structures," Ukraine's military intelligence said.
It said Russia was using the railway line to transport ammunition from a plant in the town of Chapayevsk, and that the damage would put it out of service for "a long time".
"An explosive device damaged a pillar on a rail bridge" over the Chapayevka river, Russia's official TASS agency quoted a source in the rescue services as saying.
Traffic on the line was suspended following the attack but later restored, Russian news agencies reported.
Russia's railway operator said "an intervention by non-authorised persons" caused the incident, and that no-one had been injured.
Security services in the region have opened a criminal case into "sabotage", the RIA news agency said.