Russia is still sending weapons to separatist rebels battling the Ukrainian government despite a fresh truce in the war-torn east of the country, NATO chief Jens Stoltenberg said Monday.

"The ceasefire is mainly holding. This is, of course, encouraging because that was not the case before," Stoltenberg said on the first day of a visit to Ukraine.

"But the situation is very fragile," he said at a joint press conference with Ukrainian President Petro Poroshenko at a military base.

"Russia continues to support the separatists, provide them with weapons, with different kinds of equipment, training, forces."

Kiev and the West insist that Moscow has sent troops and arms to fuel a separatist conflict that has claimed almost 8,000 lives in east Ukraine since April 2014, allegations Moscow vehemently denies.

Fighting has dropped to nearly its lowest level since the conflict began after a fresh ceasefire deal was agreed on September 1 following a series of failed truces.

Stoltenberg on Monday started a two-day visit, his first trip to the crisis-hit nation, in a show of support for Ukraine's pro-Western government.

The NATO chief insisted any attempt by separatists to hold their own unilateral local elections in the territories they control would breach a peace plan signed in February.

"They will be fake elections and they will not be recognized by any NATO ally," he told journalists.