NATO Secretary General Jens Stoltenberg says the United States and European countries bear the responsibility of promptly delivering military assistance they have promised to Ukraine.

Stoltenberg and Ukrainian President Volodymyr Zelenskyy spoke to reporters after holding talks in Kyiv on Monday.

Zelenskyy referred to additional military support announced by the United States and other countries. He said he doesn't see any positive developments regarding timely support for his country's armed forces. He said limited supplies have begun, but that the process needs to be sped up.

Citing stalled military aid for Ukraine, Stoltenberg said the lack of ammunition has enabled Russia to push forward along the frontline.

But the NATO chief said it's not too late for Ukraine to prevail as more support is on the way.

He said that "our responsibility is to ensure these announcements are turned into real deliveries" of weapons and ammunition as soon as possible.

Ukraine faces serious shortages of ammunition and air defense missiles, resulting in Russian forces' advance on the frontlines in eastern Ukraine.