The United States has already sold Javelin anti-tank missiles to Ukraine in the past year, but there are "other systems, snipers systems, ammunition" that Washington could provide to strengthen Ukraine's forces, the general told Congress.

He said the US may have to consider boosting naval defenses in the Black Sea, after Russian forces shot at and seized three Ukrainian ships late last year as they were traversing the Kerch Strait linking the Sea of Azov with the Black Sea.

Five years after Russia annexed the Crimean peninsula, Moscow "continues to arm, train," and even "fight alongside antigovernment forces in eastern Ukraine," he said, calling its activities a breach of the 2015 Minsk agreement designed to end the conflict.

"The conflict in eastern Ukraine remains hot, with numerous ceasefire violations reported weekly," Scaparrotti said.

After a popular uprising in Ukraine in 2014 and the installation of a pro-Western leadership, Russia annexed Crimea. Kiev and European governments accuse Moscow of backing separatist groups in the east of the country in a conflict that has left some 13,000 dead.

Asked about NATO member Turkey's desire to buy the Russian-made S400 anti-aircraft defense system, the general reiterated the Pentagon's position that Washington should block the delivery of F-35 stealth fighters if Ankara goes ahead with the deal.

Washington delivered the first F-35s to Turkey in June 2018 but the planes remain in the United States while their Turkish pilots undergo training, a process which the Pentagon says could take two years.