Ukraine's President Volodymyr Zelensky has suspended all his international trips, while his country's forces have been forced to retreat, as Russia continues to pummel the Kharkiv region in an increasingly bloody offensive.

Through the night, officials had warned Ukraine was in an "extremely dangerous moment" - but later on Wednesday, Ukrainian officials admitted that Russian infantry had entered the border town of Vovchansk.

The assault keeps Ukraine's stretched and depleted forces off balance ahead of what President Zelensky has warned could be a further large Russian offensive in the coming weeks.

And the premier has been forced into cancelling foreign trips - including to Spain and Portugal - as Putin's military pressures ramp up.

Sergiy Nykyforov, Zelensky's press secretary, said the Pesident had "instructed that all international events scheduled for the coming days be postponed and new dates coordinated".

The capture of Vovchansk, three miles from the border would be Russia's most significant gain since it launched an incursion into the Kharkiv region on Friday, opening a new front and forcing Ukraine into rushing in reinforcements.

Oleksiy Kharkivskiy, Vovchansk's patrol police chief, said: "The situation is extremely difficult. The enemy is taking positions on the streets of the town of Vovchansk."

Late last night, the Ukrainian military said troops had been pulled back to "more advantageous" positions in two areas of the Kharkiv region - language which, in the past, has been used to describe Ukrainian retreats.

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It said the decision was "a consequence of enemy fire and storming action" and taken "to preserve the lives of our servicemen and avoid losses."

The capture of Vovchansk, though not of specific militarily significance, would represent a blow to Ukrainian morale.

A military spokesman had said that the situation "remains difficult" but insisted that the country's forces were "not allowing the Russian occupiers to gain a foothold" and had "partially pushed back" some of Russia's forces, while Ukraine's head of intelligence, Kyrylo Budanov, said troops had stabilised the front line.

Kyiv's forces were trying to prevent Russia from building up troops and military hardware in Vovchansk's north, the military said, adding that Russian forces were trying to regroup and dig in and had not taken any more "active" action on Wednesday.

Police remained in Vovchansk and were continuing to evacuate people, Kharkivskiy said; since Friday, almost 8,000 people have been evacuated from the town and nearby border areas.

Despite the setbacks in the northeast, US Secretary of State Antony Blinken - currently in Kyiv - announced the US would be dishing out an additional $2billion in military financing to Ukraine, adding that the fund will provide weapons "today", and invest in Ukraine's industrial base.

The $2billion top-up sits alongside an existing $61billion aid package passed near the end of last month by the US Congress.

The US is rushing ammunition, armoured vehicles, missiles and air defences to Ukraine to ensure their speedy delivery to the front line, Secretary of State Antony Blinken said at a press conference this afternoon.