"Our reconnaissance tell(s) us the enemy is building up its forces, including armor," said Andrei Diachenko during training exercises for new recruits outside Azov's seaside headquarters in Urzuf, southwest of Mariupol.
The brigade is one of many that fight alongside the Ukrainian army.
"That's why we don't believe in the cease-fire and we should expect a major offensive at the end of March," he added.
The remarks came as Ukraine's military said there were 46 attacks on Ukrainian positions in three separate clashes, but its forces suffered no casualties in the 24 hours ending Saturday morning.
Ukraine said it is completing a pullback of heavy weapons under the terms of a peace agreement aimed at ending the war with pro-Russian separatists that has killed more than 6,000 people.
An internationally brokered agreement calls for both sides to create a buffer zone by pulling back heavy weapons.
Saturday was the deadline for the pullbacks to be completed.
A USA TODAY reporter saw about a half-dozen pairs of Ukrainian trucks loaded with rockets and launch tubes idling on the side of the road, apparent evidence that some weapons have been pulled back.
Rebel official Denis Pushilin said Saturday that separatists had completed their pullback ahead of schedule, and another top rebel, Eduard Basurin, said even some mortars not covered under the agreement were being pulled back.
Diachenko, whose claims could not be confirmed, described the Russian-backed separatists as "just moving tanks from one place to another."
"Even if they pull back, they can easily back to their original positions within a few hours or sooner," he added.
In addition, 1,000 new fighters and 20 armored personnel carriers were observed moving across the Russian border into Ukraine to the towns of Novoavosk and Telmanove, Diachenko said.
If Russian forces moved on Mariupol, "it would very very difficult," Diachenko said.
Mariupol, a city of about a half-million residents before the war began, is the biggest Ukrainian-held city in the southeastern sector close to the Russian border.
While smoke rose from its steel plant and some businesses and hotels remain open, the city was eerily quiet Saturday.
At the beach beside the steel plant, Nastya Kolotiy, a young woman building sandcastles with her husband and toddler, said she hopes for peace in her city.
"Whatever happens, love and friendship will win," Kolotiy said.
In Urzuf, a few miles to the southwest, new recruits trained with live ammunition at a former military base.
They chanted "I love Azov" while doing push-ups in full body armor, shot at paper targets and practiced advancing and covering each other across a grassy field.
The men included recruits from across Ukraine, Russia and Belarus.
Source: USA Today