Ukraine has broadened a request for cluster bombs from the US to include a weapon that it wants to modify to drop the anti-armour bomblets it contains on Russian forces from drones, two US politicians say.
Kyiv has urged members of Congress to press the White House to approve sending the weapons but it is by no means certain that the Biden administration will sign off on that.
Cluster munitions, banned by more than 120 countries, normally release large numbers of smaller bomblets that can kill indiscriminately over a wide area, threatening civilians.
Ukraine is seeking the MK-20, an air-delivered cluster bomb, to release its individual explosives from drones, said US Representatives Jason Crow and Adam Smith, members of the House of Representatives armed services committee.
That is in addition to 155mm artillery cluster shells Ukraine has already requested, they said.
They said Ukrainian officials urged the US at last month's Munich Security Conference to press for White House approval.
Ukraine hopes cluster munitions will give it an edge in the grinding fight against Russian forces in the country's east.
The Ukrainian government has said publicly that it wants US cluster munitions. The petition for MK-20s — also known as CBU-100s — has not been reported previously.
A National Security Council spokeswoman said that while Ukraine and the White House "closely co-ordinate" on military aid, she had no "new capabilities to announce".
Ukraine wants the artillery rounds to halt the kinds of "human wave" attacks that Russia has mounted in its months-long drive to overrun the ruined eastern city of Bakhmut, the politicians said.
Each shell disperses 88 submissiles.
The MK-20 is delivered by aircraft. It opens in mid-flight, releasing more than 240 dart-like submunitions, or bomblets.
The Ukrainian military believes these submunitions "have better armour-piercing capability" than the weapons it has been dropping from drones, said Mr Smith, the top Democrat on the armed services committee.
Ukraine, battling an enemy with more manpower and weaponry, has used drones extensively for surveillance and dropping explosives on Russian forces.
Mr Crow, a Democrat and US Army veteran, said he might support giving the MK-20 with assurances that Ukrainians would remove the bomblets and "use them in a non-cluster employment".
Production of the MK-20s ended years ago, but the US military retains a stockpile of the Cold War-era weapon.
Republican Senator Lindsey Graham, who also took part in last month's conference, confirmed that Ukrainian officials in Munich urged US legislators to press the White House to provide Kyiv with cluster munitions.
Mr Graham said he would do so this week.
The congressional aide said Ukrainian officials have also privately been lobbying legislators in Washington to press for White House approval.
"That's not going to happen," Mr Smith said, referring to Biden administration approval.