The UK has offered to conduct "logistics operations" to support the delivery of the donations, a defence source said.

British defence secretary, Ben Wallace, organised a donor conference of military aid on Friday evening, which was held virtually.

Some 25 nations were on the call and all said they would continue to contribute aid, some saying for the first time their support will include lethal assistance. NATO was also represented.

Read latest updates on Ukraine invasion

In addition, two other countries that could not make the conference said separately that they would donate support, Sky News understands.

The defence source said: "The defence secretary held a virtual donor conference last night with more than 25 countries, including the US and Canada and some countries outside NATO, coordinating their support to Ukraine.

"They will continue to give humanitarian and military support, which includes ammunition and anti-tank weapons, and the UK has offered to conduct logistics operations to support the delivery of donations."

The assistance includes lethal aid, like ammunition, anti-tank weapons and anti-air weapons, and non-lethal aid such as medical supplies.

The Netherlands has announced that it will supply 200 Stinger missiles - man-portable air defence systems - to Ukraine.

It is understood that the aid is able to reach the Ukrainian military even as they battle Russian forces on multiple fronts.

Sky News also understands that countries that had previously been against sending weapons to help the Ukrainian armed forces have now decided to do so following Russian President Vladimir Putin's decision to invade his neighbour.

The donating nations include countries that are not part of the NATO defence alliance.

Read more: Russian invasion of Ukraine mapped

Key developments:

• According to the UK's Ministry of Defence, Russian forces have made advances throughout the day - "but continue to face strong resistance from the Ukrainian Armed Forces"
• Ukraine also "retains control of key cities"
• China has abstained from voting on a draft UN Security Council resolution that would have deplored Moscow, in what will be seen as a diplomatic win for the West
• The UK is going to impose direct sanctions on Vladimir Putin and Russian Foreign Minister Sergei Lavrov imminently. The EU also plans to freeze their assets, and the US intends to do the same

Heavy gunfire and explosions have been heard in Kyiv overnight and fighting broke out in various cities in Ukraine as the invasion enters its third day.

Clashes are also under way in the cities of Mariupol, Kherson, Mykolaiv and Odessa, an adviser to the Ukrainian president's office said.

Russia's military said it has taken over Melitopol, a city in the southern Zaporizhzhia region, but UK armed forces minister James Heappey told Sky News it is "still in Ukrainian hands".

Ukrainian President Volodymyr Zelenskyy posted a video message on Saturday morning in which he promised their troops would keep fighting and dismissing as "fake news" claims he had asked them to give up.

In a message posted on social media on Friday night, he urged his fellow countrymen and women to "persevere", adding, "this night the enemy will use all its forces to break our resistance - vile, cruel and inhuman. Tonight they will storm."

The additional aid comes after NATO announced it will deploy more troops to eastern Europe.

"No one should be fooled by the Russian government's barrage of lies," the alliance's 30 leaders said in a joint statement after a virtual summit chaired by Secretary-General Jens Stoltenberg on Friday.

"We are now making significant additional defensive deployments of forces to the eastern part of the alliance," the statement said.

Ukraine is not part of the alliance and other members are therefore not obliged to defend it if it is attacked.

:: An earlier version of this story stated 28 countries had agreed to send more support. This has since been clarified to 27.