Updated at 10:15 a.m. ET

A main leader of Russian-backed rebels in eastern Ukraine reportedly says the separatists have launched an attack on the port city of Mariupol, where rocket fire killed at least 15 people in an open-air market and residential area.

"Today an offensive was launched on Mariupol. This will be the best possible monument to all our dead," Alexander Zakharchenko was quoted as saying by Russia's RIA news agency.

The remark comes amid a resumption of intense fighting by the separatists as a five-month cease-fire agreement with Kiev appeared to be all but finished.

The separatists launched rocket attacks on the Ukrainian government-held Sea of Azov port of Mariupol, where 15 people were reportedly killed and 46 injured in an open-air market.

As the AP explains, Mariupol is the main city between mainland Russia and the Russia-annexed Crimean Peninsula.

"Heavy fighting in the region in the autumn raised fears that Russian-backed separatist forces would try to establish a land link between Russia and Crimea.

"Rebel forces have positions within 10 kilometers (six miles) from Mariupol's eastern outskirts."

On Friday, Zakharchenko "vowed to push Ukrainian soldiers out of the area and said insurgents would not take part in any more cease-fire talks," The Associated Press reports.

The rebels on Friday also launched a push to retake the shattered airport in Donetsk, which has become a focal point of fighting in recent months.

The renewed fighting appears to spell the end of a cease-fire agreement reached in September with the help of Belarus.

As The New York Times notes:

"Civilians are being hit by deadly mortars at bus stops. Tanks are rumbling down snowy roads in rebel-held areas with soldiers in unmarked green uniforms sitting on their turrets, waving at bystanders — a disquieting echo of the "little green men" whose appearance in Crimea opened this stubborn conflict in the spring.""The renewed fighting has dashed any hopes of reinvigorating [the] cease-fire ... It has also put to rest the notion that Russia's president, Vladimir V. Putin, would be so staggered by the twin blows of Western sanctions and a collapse in oil prices that he would forsake the separatists in order to foster better relations with the West."Copyright 2016 NPR. To see more, visit http://www.npr.org/.