A third Russian airfield was set ablaze on Tuesday by a drone strike, a day after Ukraine demonstrated its ability to penetrate hundreds of kilometers into Russian air space with attacks on two bases despite Washington modifying its HIMARS rockets to prevent Ukraine from firing them into Russia territory.

Officials in the Russian city of Kursk, around 90 kilometers north of the Ukraine border, released pictures of black smoke above an airfield in the early hours of Tuesday after the latest strike. The governor said an oil storage tank had gone up in flames, but there were no casualties.

Russia on Monday also confirmed that it had been hit hundreds of kilometers from Ukraine by Soviet-era drones at Engels air base, home to Russia's strategic bomber fleet, and in Ryazan, a few hours' drive from Moscow.

Ukraine has not claimed responsibility for the strikes, but The New York Times, citing a senior Ukrainian official, said the drones were launched from Ukrainian territory, and at least one of the strikes was carried out with the help of special forces close to the base.

In Washington, U.S. State Department spokesman Ned Price said on Tuesday that there was no confirmation the strikes were carried out by Ukraine, and that the U.S. was not enabling and encouraging Ukraine to strike beyond its borders. He added that the U.S. has not provided Ukraine with weapons that are to be used within Russia.

"We have been very clear that these are defensive supplies," Price said.

Two days earlier, The Wall Street Journal (WSJ) reported that the U.S. has secretly modified the powerful HIMARS rocket launchers it sent to Ukraine to prevent it launching long-range missiles into Russia. The Ukrainian military 20 High Mobility Artillery Rocket System launchers, known as HIMARS, are a sizable stock of satellite-guided rockets with a nearly 50-mile range. These rocket systems have been employed to attack Russian command posts, logistics hubs, and munitions storage facilities on Ukrainian territory.

The report claimed that the U.S. has not provided Ukraine with long-range Army Tactical Missile System rockets, or ATACMS missiles, which have a range of approximately 200 miles (about 322 kilometers). The modified HIMARS would prevent Ukrainian forces from launching ATACMS if obtained from another source.

The WSJ also reported that the modifications reflect concerns among officials in the Joe Biden administration that their Ukrainian partner would stop upholding its pledge not to attack Russian territory with U.S.-supplied weapons. The Biden administration has gone to strike a balance between its backing for the Ukrainian military and possible escalation with Moscow.

So far, no political talks are under way to end the conflict. Moscow insists it will not negotiate unless Kyiv and the West accept its sovereignty over Ukrainian lands it claims, while Kyiv says Russia must leave all its territory.

(With input from Reuters)