Democratic lawmakers voiced concern about President Joe Biden’s decision to send cluster munitions to Ukraine as NATO countries, some of which have also spoken out against the weapons, are set to meet this week.

“No, cluster bombs should never be used. That’s crossing a line,” Representative Barbara Lee, a California Democrat, said on CNN’s State of the Union on Sunday.

Over 100 countries have agreed to ban the use of cluster munitions, in part because of the danger to civilians from unexploded bomblets. That includes many of the North Atlantic Treaty Organization’s 31 member countries, which will discuss aid to Ukraine at a NATO summit starting Tuesday that Biden is due to attend.

Biden’s team said the choice was difficult but necessary because Ukraine’s stock of ammunition is dwindling as it mounts a counteroffensive against Russia’s invasion. “I think they needed them,” Biden said in a CNN interview Friday.

Read more: Solidarity or Squabbling: Five Things to Watch at NATO’s Summit

Another source of lawmakers’ criticism is a Congressional mandate that cluster bombs sent from the US have a non-explosion rate of less than 1%. The US supply to Ukraine has a 2.5% failure rate, according to Biden’s national security adviser, Jake Sullivan.

“Look, I share this concern,” Virginia Democratic Senator Tim Kaine said on Fox News Sunday, but the US “wouldn’t be in this position” if Russia wasn’t already harming Ukrainian civilians. Ukraine has given the US “a variety of assurances” that the risk to civilians will be minimized, Kaine said.

Lee is among 19 House Democrats who criticized Biden’s decision last week, saying it “hurts the global effort to eradicate these dangerous munitions, taking us down the wrong path,” according to a statement from Congressional Progressive Caucus chair Pramila Jayapal.

A group of moderate Democrats expressed support for the administration’s stance.

Senator John Barrasso, a Republican from Wyoming backed the decision. “Russia’s been using them all along, and I think this fills a gap that the folks need in Ukraine, in their battle,” he said on Fox News Sunday.

Ukraine’s ambassador to the US, Oksana Markarova, said on CBS’s Face the Nation that there “is nothing controversial about it.” Ukraine will be “very responsible” in deploying the munitions, track where they’re used and de-mine those areas, she said.