Russia's Foreign Minister Sergei Lavrov said on Monday that U.S.-built F-16 fighter jets can "accommodate" nuclear weapons and warned that supplying Kyiv with them will escalate the conflict further.

"We must keep in mind that one of the modifications of the F-16 can 'accommodate' nuclear weapons," Lavrov said in a speech at a military base in Dushanbe in Tajikistan, according to a transcript on the ministry's website.

"If they do not understand this, then they are worthless as military strategists and planners."

White House spokesman John Kirby on Tuesday declined to discuss the nuclear capabilities of the F-16 fighters but noted that U.S. President Joe Biden has been consistent that he does not want to see the war escalate, certainly not into the nuclear realm.

"The purpose of providing advanced fighter aircraft is to help Ukraine defend itself defend its airspace and its territorial integrity period," Kirby said.

Kirby also had a message for Lavrov: "If you’re worried about Ukrainian military capabilities, then you should take your troops and leave Ukraine."

Ukraine's President Volodymyr Zelenskiy has long appealed for the F-16 jets, saying their appearance with Ukrainian pilots would be a sure signal from the world that Russia's invasion would end in defeat

Biden told G7 leaders last month that Washington supported joint allied training programmes for Ukrainian pilots on F-16 fighters.

U.S. National Security adviser Jake Sullivan, however, said there was no final decision on Washington sending aircraft.

(Reporting by Lidia Kelly in Melbourne and Nandita Bose in Washington; Editing by Michael Perry and Marguerita Choy)