WASHINGTON, June 20 (Reuters) - A U.S. citizen was killed in combat in Ukraine last month, according to an obituary and the State Department, after he joined thousands of foreign fighters who have volunteered to help Ukraine fend off invading Russian forces.

Stephen Zabielski, 52, was killed in fighting on May 15, according to an obituary published in The Recorder, an upstate New York newspaper, earlier this month. Media reports of his death circulated on Monday.

Zabielski, who was from New York and had moved to Florida in recent years, is survived by his wife, five stepchildren, and a grandchild, among other family.

In a statement, a State Department spokesperson confirmed Zabielski's death in Ukraine and said the agency has been in touch with his family and provided "all possible consular assistance."

The spokesperson's statement repeated earlier warnings that U.S. citizens should not travel to Ukraine because of the conflict and the potential for the Russian government to single them out. It added that any citizens in Ukraine should depart immediately.

Despite the earlier warnings not to take up arms in Ukraine, several Americans have volunteered to fight alongside Ukrainian forces.

Russia says it is involved in a "special military operation" in Ukraine.

The news of Zabielski's death comes as several American combatants have gone missing in Ukraine.

Kremlin spokesman Dmitry Peskov on Monday confirmed that Americans Andy Huynh, 27, of Hartselle, Alabama, and Alexander Drueke, 39, of Tuscaloosa, Alabama, were detained while fighting on the Ukrainian side and said they were mercenaries who endangered the lives of Russian servicemen and should face responsibility for their actions.