WASHINGTON 

The US announced Wednesday that it will, for the first time, supply Ukraine with depleted uranium tank ammunition as Kyiv's forces press a counteroffensive to drive Russia out of occupied territories. 

The decision to supply Ukraine with the controversial anti-armor munitions comes as part of Washington's expected delivery of 31 M1 Abrams tanks, the US' premier main battle tank. They are expected to begin arriving in Ukraine "soon," according to the White House.

Russia has warned it would treat any delivery of depleted uranium armor-piercing ammunition as an escalation in the conflict. Hours before the official announcement, the Kremlin launched an artillery strike on a market in eastern Ukraine that killed at least 16 civilians.

It is unclear from a Pentagon statement how many depleted uranium rounds the US is sending to Ukraine. The UK previously sent Ukraine the controversial arms for its Challenger 2 tanks.

The anti-tank rounds are part of a new $175 million military aid package announced as Secretary of State Antony Blinken pays a two-day visit to the war-torn nation.

The new tranche includes equipment to bolster Ukraine's air defense systems, HIMARS long-range rocket ammunition, 155mm and 105mm artillery rounds, 81mm mortar systems and rounds, anti-tank TOW missiles, Javelin and AT-4 anti-armor systems and more than 3 million rounds of small arms ammunition.

Blinken arrived in the Ukrainian capital by train for an unannounced visit in which he will meet senior Ukrainian officials. The visit is his fourth to Ukraine since Russia began its war against its eastern European neighbor in February 2022.

Blinken said ahead of a closed-door meeting with Ukrainian President Volodymyr Zelenskyy that the Biden administration is "determined" to remain in lockstep with Kyiv, and said the US sees "the important progress that’s being made now in the counteroffensive, and that’s very, very encouraging."

Blinken said he would discuss efforts "to ensure that we are maximizing the efforts that we’re making and other countries are making for the immediate challenge of the counteroffensive as well as the longer-term efforts to help Ukraine build a force for the future that can deter and defend against any future aggression."