An armed man has taken a number of people hostage on a bus in the western Ukrainian city of Lutsk on Tuesday.
Shots have been heard and the bus damaged, as police officers blocked off the city centre.
The suspect, identified as 44-year-old Russian-born Maksym Kryvosh, through social media is demanding that senior Ukrainian officials publish statements saying that they were terrorists.
He also threatened to detonate another bomb in a crowded place.
Deputy interior minister Anton Gerashchenko said Kryvosh had been convicted twice and spent about 10 years in prison.
"We are in full control of the situation. I know all the details. I am talking with our specialists who are in Lutsk. Professionals are working, doing everything to free our hostages," Ukrainian President Volodymyr Zelenskiy's press service quoted him as saying.
Photos showed a small bus parked in the middle of an empty street. Two windows of the bus were smashed and other windows were covered with curtains.
Pope Francis' funeral will be held on Saturday in St. Peter's Square, Roman Catholic cardinals decided on Tuesday, setting the stage for a solemn ceremony that will draw leaders from around the world.
Cardinals gathered at the Vatican on Tuesday to plan Pope Francis' funeral, which leaders from around the world will attend ahead of a conclave next month to elect a new head of the Roman Catholic Church.
Hundreds of Filipinos gathered at a solemn Mass held for Pope Francis on Tuesday, following his passing that has stirred deep sorrow among Catholics around the world, many of whom saw him as a humble and compassionate leader.
Russian President Vladimir Putin proposed on Monday bilateral talks with Ukraine for the first time since the early days of the war, and his Ukrainian counterpart Volodymyr Zelenskyy said Kyiv was eager to discuss a halt to attacks on civilian targets.
Pope Francis, the first Latin American leader of the Roman Catholic Church, died on Monday after suffering a stroke and cardiac arrest, the Vatican said in a video statement.