British Defense Secretary Michael Fallon has called on all NATO members to commit to increase military spending every year.
Fallon made the comments during a joint news conference with his US counterpart James Mattis in London on Friday.
"Secretary Mattis and I have agreed that others must now raise their game, and those failing to meet the 2 percent commitment so far should at least agree to year on year real terms increases," Fallon said.
The British minister said the UK and the US are providing reassurance to their eastern European allies against what he called Russian aggression.
He said the two countries continue sending troops and military hardware to Eastern Europe to this end. For his part, Mattis said that mutual assistance is the bedrock of the NATO alliance.
NATO has previously announced that its current priority is for its members to raise the amount they spend on military.
Earlier, US Secretary of State Rex Tillerson also urged NATO members to increase their military spending and to do more to fight terrorism.
The United States is urging NATO allies to increase their spending towards the minimum criterion of two percent of their economic output by the end of the year.
US President Donald Trump, who once denounced the Western military alliance as "obsolete", has said NATO members should either pay for US military support or rely on their own military might at the time of war.