KIEV, Sept. 8 - President Petro Poroshenko visited the eastern port of Mariupol on Monday and promised to deal a "crushing defeat" to pro-Russian rebels massed on the edge of town if they tried to advance in violation of a ceasefire agreement, wire services reported.

"I have ordered (the military) to secure the defense of Mariupol with howitzers, multiple rocket launchers, tanks, anti-tank weapons and air cover," Poroshenko told a crowd of steel workers in the port on the Sea of Azov near the Russian border.

The ceasefire, which took effect on Friday evening, is part of a peace plan intended to end a five-month-old conflict the United Nations' human rights envoy said had killed more than 3,000 people. It has also caused the sharpest confrontation between Russia and the West since the Cold War.

The truce was largely holding on Monday, though each side accused the other of sporadic shelling, including in Mariupol, a city of about half a million, shortly after the president's arrival there.

The Organization for Security and Cooperation in Europe (OSCE), which is monitoring the ceasefire, urged the two sides to seek a political "breakthrough", though they remain far apart on the future of eastern Ukraine, home to much of the country's heavy industry. The rebels refuse to accept rule from Kiev.

"Mariupol was, is and will be Ukrainian," Poroshenko declared.

"The enemy will suffer a crushing defeat," said Poroshenko, who agreed to the ceasefire and a wider peace plan after the rebels - backed, Kiev says, by Russian firepower - made sweeping battlefield gains. Russia denies involvement.

In the earlier days of the uprising, rebels seized control of part of Mariupol, occupying some buildings including a police station. Some offices were badly damaged or burnt down. Since the rebels were driven out by Ukrainian forces, sentiment appears to have swung more in favor of the government.

Residents built fortifications around the town, whose port is vital for Ukraine's steel exports, and set up a militia. Shops have reopened and Ukrainian flags are widely visible.

Mariupol was the scene of fierce fighting before the ceasefire, when rebels advanced in an attempt to retake it, and also saw the most serious violation of the ceasefire on Saturday night when government forces there came under artillery attack.

A woman was killed and four people injured in that shelling.

The cities of Donetsk and Luhansk, strongholds of the rebellion, remain in rebel hands.

Poroshenko, who received a warm welcome in Mariupol, said the rebels had so far handed over about 1,200 prisoners-of-war to the Ukrainian side under the terms of the ceasefire accord.

OSCE chair Switzerland described the ceasefire as "shaky" and said the next few days would be crucial.

Swiss President Didier Burkhalter said the truce alone was not sufficient, adding: "The different actors must really push for a (political) breakthrough."

After his trip to Mariupol, Poroshenko said a number of NATO countries had agreed on the direct supply of arms to Ukraine during the alliance's annual summit in Wales.

"(We) managed to agree with a series of NATO countries on direct deliveries of modern weapons which will help us defend ourselves and win," said a statement on the president's website.

A senior aide to Poroshenko said on Sunday Kiev had agreed in Wales on the provision of weapons and military advisers from five NATO states, but four of the five swiftly denied any such deal had been reached.

U.S. Defense Secretary Chuck Hagel said in Turkey he was not aware of any "secret deal that was made in Wales about supplying lethal weapons to the Ukrainians".

Earlier, Ukrainian military spokesman Andriy Lysenko told a news briefing in Kiev that Ukrainian forces were observing the ceasefire except in self-defense and had remained in their positions since Friday evening. (rt/ez)