Western nations have so far been successful in delivering arms to Ukraine, but this may become harder in the coming days, US Deputy Secretary of State Wendy Sherman said Monday.

"I think it has been extraordinary the amount of arms supplies that are getting into Ukraine even under the most difficult of circumstances," she told a media briefing in Madrid.

"The international community has been tremendously responsive and has found ways to get the material in. That may become harder in the coming days and we will have to find other ways to manage this," she said without elaborating.

Weapons, ammunition and funds have poured into Ukraine from Western allies since Russia invaded the country on February 24.

The United States last month authorised $350 million of military equipment -- the largest such package in US history -- to help the Ukrainian government fight off the invasion.

The European Union, meanwhile, has agreed to finance the purchase and delivery of arms to Ukraine totaling 450 million euros ($500 million).

Kyiv has urged the West to boost its military assistance to the besieged country, including warplanes, with President Volodymyr Zelensky pleading for Eastern European neighbors to provide Russian-made planes that his pilots are trained to fly.

"It is critical that what we send in is what President Zelensky asks for because he knows what his military needs most," said Sherman, who is on a week-long trip to Turkey, Spain, and North Africa.