Delegates from Ukraine and Russia have sought a ceasefire at the negotiating table for a fourth time. The latest round of talks took place Tuesday in Istanbul. Turkish media say it's unclear if the delegates will meet for a second day.

Before the talks, both sides signaled their readiness to consider a new framework that would ensure Ukraine's security without NATO membership.

Russia wants guarantees Ukraine will not join the security alliance. Ukraine wants an immediate halt to the bloodshed, and the withdrawal of Russian forces.

The sticking points include Moscow's demand that Crimea be recognized as Russian territory. The Kremlin also wants the eastern regions of Ukraine effectively controlled by pro-Russia separatists to be recognized as independent states.

For weeks, Russian troops have been merciless in their shelling of Mariupol, a strategic port city in the southeast. They're still aiming to take full control.

The mayor's office told local media that 5,000 people have been killed, including more than 200 children.

Elsewhere, concerns are rising about the Institute of Physics and Technology in Kharkiv. Russian troops have once again attacked the facility, which handles nuclear material. Authorities are reporting serious damage. They say the actual extent is unknown due to fierce fighting.

Military analysts say the Russians may be seeking to pin down Ukraine's forces in the east to free up the north and south.

But the Ukrainians are not rolling over. They are said to have retaken control of Trostyanets -- a town in the northeastern region of Sumy.

A man who lives in Ukaranian city of Trostianets said, "We are in a humanitarian crisis, with no food or water. But there have been no missiles for three days. The situation appears to be improving."

Ukrainian officials say they have also retaken key areas around the capital Kyiv.