Ukraine accuses Russia of sending mercenaries into rebel-held territories.

KYIV, Ukraine — Ukraine’s military intelligence service said on Friday that Russia was sending mercenaries into rebel-held territories in eastern Ukraine, along with tanks, mobile artillery units and 7,000 tons of fuel, raising fears of military escalation in the region.

The mercenaries are being deployed to fighting units in Luhansk and Donetsk “after undergoing a course of intensive training,” the agency, known as the G.U.R., said in a statement on Facebook. The precise number of mercenaries is unclear, but a senior Ukrainian security official said it was at least 1,000.

The official said that they were being sent in small groups, and that the deployment had been going on for about three weeks.

This month, the White House accused Moscow of sending saboteurs into eastern Ukraine, potentially to set off a provocation that could give President Vladimir V. Putin of Russia the pretext to order into action the roughly 127,000 troops he has deployed near Ukraine’s borders.

The Kremlin has denied that there are plans to attack Ukraine and pushed back against the American intelligence assessment that it had sent saboteurs to the country.

“You claim that we are preparing to attack Ukraine, but we have explained on many occasions that this is not the case,” Sergei V. Lavrov, Russia’s foreign minister, said at a news conference on Friday after his meeting with Secretary of State Antony J. Blinken in Geneva. Mr. Lavrov accused Western countries of destabilizing the situation by sending weapons and military advisers to Ukraine.

How military action by Russia might commence is anyone’s guess, but the tense conditions in Ukraine’s east could easily supply the spark. Separatist forces seized the breakaway provinces from Ukraine in 2014, aided by Russian arms, equipment and troops. After a series of vicious battles, the war has settled into a grinding stalemate, with Ukrainian forces and rebels dug into trenches along both sides of the so-called line of contact.

On Friday, the Kremlin-run news agency RIA reported that Ukrainian forces had moved several rocket-launching systems close to the conflict zone in preparation for a potential attack. The report echoed statements by senior Russian officials who have repeatedly accused Ukraine of preparing to retake the rebel territories by force.

The senior Ukrainian security official denied that preparations for attack were underway, and said that all heavy artillery and rocket systems remained farther than about 20 miles from the conflict line as required by a 2015 cease-fire agreement.