LONDON — Joe Biden and Rishi Sunak were all smiles Monday as they met in London — but concern about the United States’ decision to send cluster bombs to Ukraine lingered.

The U.K. prime minister told the U.S. president Monday that Britain will “stand by our obligations” under a treaty banning the production or use of the controversial munitions, according to Sunak’s official spokesperson.

Publicly, the two leaders — meeting for the sixth time since Sunak became prime minister last year — demonstrated a united front ahead of this week’s NATO summit in Vilnius.

Speaking in front of reporters at Downing Street, Biden said he “couldn’t be meeting with a closer friend and a greater ally.”

In a sit-down that ran to just 40 minutes, and saw the pair drink tea in the garden of 10 Downing Street, the U.S president and British PM reaffirmed what Biden called “rock solid” relations between the two countries.

“We stand as two of the firmest allies in that alliance and I know we’ll want to do everything we can to strengthen Euro-Atlantic security,” Sunak said.

But the U.S. decision to arm Ukraine with cluster bombs as part of its latest weapons package has already opened a rift among NATO countries ahead of the summit.

Sunak pointedly said over the weekend that the U.K. “discourages” use of the weapons — and his spokesperson reiterated that position in a briefing after the Biden-Sunak meeting, saying the prime minister had raised the issue directly with the U.S. president.

Britain is one of more than 100 countries who are signed up to the Convention on Cluster Munitions, which prohibits “the use, production, transfer and stockpiling of cluster munitions that cause unacceptable harm to civilians.”

“When it comes to munitions, I think the first thing to say is this was difficult choice for the U.S. that’s been forced on them by Russia’s war of aggression,” Sunak’s spokesperson told reporters.

But they added: “As the prime said over the weekend, the U.K. is a state party to the convention on cluster munitions. … They discussed the commitment the U.K. has under that convention, both not to produce cluster munitions and to discourage their use.”

Asked whether Sunak had “discharged his duty” to discourage use of the weapons in the Biden meeting, the spokesperson said: “Yes. They discussed the requirements the prime minister is under because of this convention, and the U.K. is upholding it.”

Ukraine’s aim of joining NATO will be another key theme of this week’s summit. Biden said in an interview Sunday that Ukraine was “not ready” to join the military alliance, arguing that membership could only be considered once the war is over.

Sunak’s spokesperson said Monday that the U.K.’s “longstanding position” on NATO membership for Ukraine — wholeheartedly backing Kyiv’s bid — hasn’t changed.

He said: “The prime minister believes and has said previously that Ukraine’s rightful place is in NATO and we want to work with the U.S. and our allies on the pathway for Ukraine to join that alliance.”