Ukraine says it has halted a Russian offensive in northeastern Ukraine after months of fierce fighting.

"Today we can say that our forces have completely thwarted the Russian offensive operation (in the Sumy area)," President Volodymyr Zelensky wrote on the Telegram platform after consultations with the military leadership, including the Supreme Commander-in-Chief of the Armed Forces Oleksandr Syrsky.

Fighting continues on the border, but Russian forces can no longer attack, Zelensky noted.

In June, Russian President Vladimir Putin reiterated that Russian troops should capture a buffer zone of about 10 kilometers in the Ukrainian border region of Sumy, in order to prevent a Ukrainian offensive, including in Russia's western Kursk region.

According to Ukrainian military observers, Russian troops still control more than 200 square kilometers of territory in the Sumy region.

Meanwhile, US President Donald Trump announced on Saturday that he was ready to impose serious sanctions on Russia, but he set two conditions for this - NATO countries must also impose sanctions on Russia and must stop buying Russian oil.

"I am ready to impose serious sanctions on Russia when all NATO countries agree and start doing the same themselves, and when all NATO countries STOP BUYING OIL FROM RUSSIA."

"This greatly weakens your bargaining position and bargaining power against Russia. In any case, I am ready to 'take action' whenever you are. All you have to do is say when," the US president wrote.

Trump said that this step would help bring an end to the conflict, and NATO countries could also impose 50-100 percent tariffs on China, which would be lifted after the war in Ukraine ends.

"China has strong control and even a stranglehold on Russia, and these powerful tariffs would break that stranglehold," he said.