Russian Foreign Minister Sergei Lavrov will travel to Turkey on Wednesday for talks with his Ukrainian counterpart Dmytro Kuleba, the TASS news agency cited the Russian foreign ministry as saying.

The meeting was to take place on the sidelines of a summit being hosted by Turkey, a Nato member nation, but no further details were announced.

Meanwhile, Ukrainian forces were bolstering defences in key cities today as Russia's advance faltered amid fierce resistance in some areas, the general staff of Ukraine's armed forces said, while the strategic port city of Mariupol remained encircled as a humanitarian crisis grew.

Across the country, thousands of people are thought to have been killed, both civilians and soldiers, in nearly two weeks of fighting.

Russian forces have seen their advances stopped in certain areas including around Kyiv by fiercer resistance than expected from the Ukrainians.

Ukraine's general staff said in a statement that it was building up defences in cities in the north, south and east and that forces around Kyiv are resisting the Russian offensive with unspecified strikes and holding the line.

In the northern city of Chernihiv, Russian forces are placing military equipment among residential buildings and on farms, the Ukrainian general staff said. And in the south, it said Russians dressed in civilian clothes are advancing on the city of Mykolaiv.

Ukrainians urged to take 'bomb shelters'

In Kyiv, back-to-back air alerts on Wednesday morning urged residents to get to bomb shelters as quickly as possible over fears of incoming Russian missiles. An all-clear was given for each alert soon afterward.

Such alerts are common, though irregular, keeping people on edge. Kyiv has been relatively quiet in recent days, though Russian artillery has pounded the outskirts.

Kyiv regional administration head Oleksiy Kuleba said the crisis for civilians was growing in the capital, with the situation particularly critical in the city's suburbs.

Russia is artificially creating a humanitarian crisis in the Kyiv region, frustrating the evacuation of people and continuing shelling and bombing small communities, he said.

"More than 2 million people have now fled Ukraine," according to the United Nations.

As Moscow's forces have laid siege to Ukrainian cities, the fighting has thwarted attempts to create corridors to safely evacuate civilians.

One evacuation did appear successful, with Ukrainian authorities saying on Tuesday that 5,000 civilians, including 1,700 foreign students, had been brought out via a safe corridor from Sumy, an embattled northeastern city of a quarter-million people.

Russia's Defence Ministry spokesman Major General Igor Konashenkov said the forces had thwarted a large-scale attack plot in the east, citing in a televised statement what he claimed was an intercepted Ukrainian National Guard document.

The special military operation of the Russian armed forces, carried out since February 24, pre-empted and thwarted a large-scale offensive by strike groups of Ukrainian troops on the Luhansk and Donetsk Peoples Republics, which are not controlled by Kyiv, in March of this year, Konashenkov said.