Ukrainian officials said Thursday they are -- once again -- trying to evacuate thousands of civilians through humanitarian corridors under temporary cease-fires, if they will hold.

So far, evacuations in some cities are managing to go ahead while others are already failing, as Ukrainian officials accuse Russian forces of blocking or deliberately firing on the routes.

Ukrainian Deputy Prime Minister Iryna Vereshchuk said seven humanitarian corridors -- from several besieged cities as well as areas north of Ukraine's capital -- have been agreed upon with Russia for Thursday. The question is whether Russian forces will uphold their end of the deal.

An attempt to evacuate the areas north of Kyiv was underway, with buses trying to reach the towns of Irpin, Bucha, Hostomel and Borodyanka. The Kyiv region's administration told ABC News that they were able to evacuate 15,000 people -- primarily from Irpin and the town of Vorzel -- but Russian troops refused to allow access to Bucha, Hostomel or Borodyanka.

Ukrainian officials were also hoping an evacuation would take place Thursday from Mariupol, the hard-hit southeastern port city where the humanitarian situation is arguably the worst, after Russian airstrikes destroyed a children's hospital and maternity ward there on Wednesday. But Petro Andrushenko, an adviser to Mariupol's mayor, told ABC News that no evacuation can happen Thursday because Russian warplanes have launched multiple airstrikes in the city center since the early morning. At least four aircraft had been spotted and around a dozen bombs had fallen, according to Andrushenko.

He said it was "physically impossible" right now to evacuate people "under bombs and bullets." Nevertheless, there were reports that buses have set off in an attempt to reach Mariupol.

Russia has made clear that, despite the alleged humanitarian corridors, it is continuing its operation to "liberate" Mariupol.

Meanwhile, thousands of people are independently leaving Ukraine's second-largest city, Kharkiv, without a humanitarian corridor because the trains are still running and there are ways out of the besieged city.

-ABC News' Patrick Reevell

Ukraine's second-largest city, Kharkiv, was hit with several powerful airstrikes overnight that killed at least four people, local authorities said Thursday.

Russian bombardment have destroyed 280 civilian buildings in Kharkiv, including schools and kindergartens, since Russia began invading Ukraine on Feb. 24, according to the regional interior ministry department in Kharkiv.

Kharkiv has come under heavy attacks as Russian forces try to seize the city.

-ABC News' Patrick Reevell

The Russian military alleged Thursday that its forces have managed to capture more of the outer neighbourhoods of Mariupol, in southeastern Ukraine, saying the operation to "liberate" the strategic port city is ongoing.

The claim came a day after a Russian airstrike destroyed a children’s hospital and maternity ward in Mariupol, where heavy fighting has been taking place in recent days.

Local authorities in the besieged city have accused Russian forces of waging a "medieval siege" against them.

-ABC News' Patrick Reevell

The top diplomats from Russia and Ukraine are meeting now in Antalya, Turkey.

Russian Foreign Minister Sergey Lavrov and Ukrainian Foreign Minister Dmytro Kuleba arrived in the southern Turkish resort on Thursday morning ahead of the meeting -- the highest level talks between their two countries since Russia invaded Ukraine on Feb. 24.

Both officials first met separately with their Turkish counterpart, Mevlut Cavusoglu, upon arrival. Cavusoglu is expected to attend their talks.

Lavrov and Kuleba are expected to talk for about 90 minutes. They will hold separate press conferences afterwards.

-ABC News' Engin Bas and Joe Simonetti