KIEV, Ukraine -- Army troops have penetrated deep inside a rebel-controlled city in eastern Ukraine in what could prove a breakthrough development in the four-month-long conflict, the Ukrainian government said Sunday.

However, the military acknowledged that another one of its fighter planes was shot down by the separatists, who have been bullish about their ability to continue the battle and have bragged about receiving support from Russia. An Associated Press reporter spotted a column of several dozen heavy vehicles, including tanks and at least one rocket launcher, rolling through rebel-held territory on Sunday.

Ukraine’s national security council said government forces captured a district police station in Luhansk on Saturday after bitter clashes in the Velika Vergunka neighborhood.

Weeks of fighting have taken their toll on Luhansk, which city authorities say has reached the verge of a humanitarian catastrophe. The siege mounted by government forces has ground delivery of basic provisions to a halt and cut off power and running water.

Although rebel forces have regularly yielded territory in recent weeks, they have continued to show formidable fighting capabilities.

Ukrainian military spokesman Oleksiy Dmitrashkovsky said Sunday that the separatists shot down a Ukrainian fighter plane over the Luhansk region after it launched an attack on rebels. The pilot ejected and was taken to a secure place, he said. Another military spokesman, Andriy Lysenko, later said that the status of the pilot was still being clarified.


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The column of armored vehicles was spotted southeast of Luhansk outside a town very close to the Russian border and was heading west, deeper into rebel-held territory. It was unclear whether the column had come from Russia. Among the armored vehicles was a Strela-10, a short-range surface-to-air missile system capable of hitting targets up to 3,500 meters (11,500 feet.)

The area is just across the border from where a large Russian aid convoy is poised to cross with supplies intended for Luhansk and other afflicted zones.

Part of the aid convoy headed to the frontier crossing on Sunday, but the 16 white trucks then stopped. The convoy of nearly 270 vehicles has been marooned for days in a town near the border amid objections from Ukraine, which initially complained that the mission was not authorized by the International Committee for the Red Cross.

The Red Cross, which would have responsibility for distributing the aid, on Saturday said the main holdup was a lack of security guarantees from all sides in the conflict.

A large X-ray machine was brought to the Russian crossing point in the afternoon, and Paul Picard, the head of a border-monitoring mission from the Organization for Security and Cooperation in Europe, said it would be used to inspect the cargo.

As the status of the Russian aid convoy remained uncertain, the foreign ministers of Ukraine, Russia, Germany and France met in Berlin late Sunday to discuss ways to end the crisis.

German Foreign Minister Frank-Walter-Steinmeier said ahead of the meeting that the search for a political solution mustn’t be neglected even as efforts are made to provide humanitarian aid to civilians in the rebel-held cities of Luhansk and Donetsk.

"Today’s news shows that things could get worse," he told reporters. "If we aren’t very careful ... we may slide further toward a direct confrontation between Russian and Ukrainian forces."

But Steinmeier lamented that previous accords reached in Geneva and Berlin had failed to hold, and added: "There’s no guarantee that today’s talks will result in the success we’re looking for." Officials familiar with the talks, who asked not to be named because they weren’t authorized to brief reporters until the meetings were over, also dampened any expectations of a major breakthrough.

The four diplomats began the evening with a demonstration of cordiality for the cameras, walking together around a tranquil park on the shores of Berlin’s Lake Tegel before retreating into a government villa for closed-doors discussions.

Fighting, including frequent shelling, is also affecting Donetsk, Ukraine’s main rebel-held city. Ten civilians have been killed and eight wounded in the past 24 hours, city authorities reported Sunday.

The leader of the self-proclaimed separatist government in the Donetsk region, Alexander Zakharchenko, has boasted that his forces have been bolstered by 1,200 fighters who underwent training in Russia and were brought in at a "crucial moment." In a video of his speech that was posted online over the weekend, he said the fighters have 150 armored vehicles, including 30 tanks, and have gathered near a "corridor" along the Russian border. Zakharchenko did not specify whether the armored vehicles had also come from Russia.

Russian President Vladimir Putin’s spokesman, Dmitry Peskov, on Sunday denied that Russia had supplied any armored vehicles to the separatists.

Lysenko, the military spokesman, said the government had information that separatists had received reinforcement from Russia, but added that there is evidence rebels are complaining about not receiving some of the equipment they have been promised.

Russia has consistently denied allegations that it is supporting the rebels with equipment or training. But Ukraine’s president on Friday said that Ukraine had destroyed a large number of military vehicles that had recently crossed from Russia.

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Jim Heintz in Kiev, Ukraine, Mstyslav Chernov in Krasnodon, Ukraine, Alexander Roslyakov in Kamensk-Shakhtinsky, Russia, and Frank Jordans in Berlin contributed to this report.