Ukraine carried out some of the heaviest shelling in years as it targeted the country's Russian-controlled east Thursday while Moscow slammed a U.S. plan to supply Kyiv with Patriot air defense missiles and dismissed a Christmas truce.

Alexei Kulemzin, the Russian-backed mayor of Donetsk city, claimed 40 rockets were fired from BM-21 Grad multiple rocket launchers at civilians in the city center in the early hours.

Meanwhile, Russian forces kept up shelling and air strikes along the entire eastern front line, killing one person, while two were killed in the southern city of Kherson, Ukrainian officials said.

Moscow and Kyiv are not currently engaged in talks to end Europe's biggest conflict since World War II, which is raging mainly in the east and south of Ukraine with little movement on either side.

"The Kremlin ... is seeking to turn the conflict into a prolonged armed confrontation," a senior Ukrainian officer, Brigadier General Oleksiy Gromov, told a news briefing on Thursday. He also dismissed the possibility of a truce over the festive period.

On Wednesday, Kremlin spokesman Dmitry Peskov had said a Christmas ceasefire was "not on the agenda."

Kulemzin cast the Donetsk attack as a war crime and said it was the biggest on the city since 2014, when pro-Moscow separatists seized it from Kyiv's control. Preliminary estimates showed five people had been hurt, including a child, he said.

There was no immediate Ukrainian response to his comments.

Ukraine's military General Staff said in its daily report that Moscow's focus remained on the eastern cities of Bakhmut and Avdiivka, adding that Ukrainian forces had repelled Russian attacks in the past 24 hours.

It also said Russian forces continued to strike Ukrainian troops and civilian infrastructure in the Donetsk region and in the southern areas of Zaporizhzhia and Kherson.

"The Russians fired at different areas along the entire front line all night and in the morning," the Ukrainian governor of Donetsk region, Pavlo Kyrylenko, said on the Telegram messaging app, adding there had been two air strikes on Avdiivka.

One person was killed and four injured near Bakhmut, he said, adding: "It is dangerous to stay in the Donetsk region! Evacuate in time!"

Separately, Russian shelling killed two people in the center of Kharkiv, the southern city liberated by Ukraine last month," said Kyrylo Tymoshenko, deputy head of the president's office.

Reuters was unable to immediately verify battlefield accounts from either side.

'Legitimate target'

The Kremlin earlier Wednesday said that U.S. Patriot missile defense systems would be a legitimate target for Russian strikes against Ukraine, should the United States authorize them to be delivered to support Kyiv.

Washington is finalizing plans to send the Patriot missile defense system to Ukraine, a decision that could be announced as soon as this week, three U.S. officials told Reuters on Tuesday.

Kremlin spokesman Dmitry Peskov said Patriots would "definitely" be a target for Russia, but that he would not comment on unconfirmed media reports.

The Patriot is considered to be one of the most advanced U.S. air defense systems, including against aircraft, cruise missiles and ballistic missiles. It typically includes launchers along with radar and other support vehicles.

Russia's embassy in Washington said the proposed transfer was provocative and could lead to unpredictable consequences.

"Even without providing Patriots, the United States is getting deeper and deeper into the conflict in the post-Soviet republic," the mission wrote on its Telegram channel.

"It is the United States that is responsible for the prolongation and escalation of the Ukrainian conflict."

The Pentagon says Russia's recent surge in missile strikes in Ukraine is partly designed to exhaust Kyiv's supplies of air defenses so it can dominate the skies above the country. For that reason, the United States and its allies have been delivering more air defenses for Kyiv.

For the United States, this has included NASAMS air defense systems that the Pentagon says have flawlessly intercepted Russian missiles in Ukraine. Washington has so far provided Ukraine with $19.3 billion in military assistance since the start of the conflict.