The two-week-old truce between Ukraine and pro-Russian militants fighting in the country's east is a "ceasefire in name only," NATO's top general said Saturday.

U.S. Air Force Gen. Philip Breedlove said that by enabling a free flow of weapons and fighters across the border Russia has made it nearly impossible for outsiders to determine how many of its troops are operating inside Ukraine.

Breedlove, the Supreme Allied Commander Europe, told a news conference after meeting with NATO military chiefs that he is hopeful about Saturday's announced agreement for creation of a buffer zone between Ukrainian and pro-Russian forces.

The deal reached by representatives of Ukraine, Russia, the Moscow-backed rebels and the Organization for Security and Cooperation in Europe marks an effort to add substance to the Sept. 5 ceasefire agreement that has been frequently broken by clashes.

'The ceasefire is still there in name, but what is happening on the ground is quite a different story.' — U.S. Air Force Gen. Philip Breedlove

Breedlove has put the main blame on Russia for the continuing conflict.

"So the situation in Ukraine is not good right now," he said. "Basically we have a ceasefire in name only."

Breedlove said violence levels in Ukraine, including the number of artillery rounds fired in the past few days, are as high as prior to the cease-fire.

"So the ceasefire is still there in name, but what is happening on the ground is quite a different story," he said.

Explosion rocks a plant in Donetsk

Meanwhile,  an explosion rocked a plant producing munitions and industrial explosives in Ukraine's eastern city of Donetsk on Saturday, municipal authorities said.

The blast sent flames and smoke billowing over the city, even as government forces and pro-Russian separatists prepared to create a buffer zone to separate the warring sides.

A Reuters correspondent in Donetsk, the main industrial hub in Ukraine's turbulent east, said several powerful explosions were heard in the morning.

Explosions were also heard from the direction of the main international airport, an area that government forces are still clinging on to despite attempts by rebels to loosen their control.

News agency RIA Novosti said ammunition supplies were being stored in the factory in Oktyabrsky district, which Ukraine government forces targeted.

Number of Russian troops in Ukraine has declined significantly

Breedlove said Russian forces are still operating inside Ukraine but numbers cannot be pinpointed.

"Right now the border is being maintained open by Russian forces and Russian-backed forces, and the fluidity of movement of Russian forces and Russian-backed forces back and forth across that border makes it almost impossible to understand the numbers," he said.

He said it is clear that the number of Russian troops in Ukraine has declined significantly over the past week or so, with some returning to the Russian side of the border — "which is good, except that they haven't returned home and are still available to bring their military force to bear on Ukraine, should it be desired" by Russian government leaders.

Neither Russia nor Ukraine is a member of NATO, but both share borders with NATO-member countries. Recent Russian military behaviour, including its annexation of the Crimea Peninsula of southern Ukraine earlier this year, is a major worry inside the U.S.-led alliance.

With files from Reuters