A 40-mile convoy of Russian tanks and other vehicles threatened Ukraine’s capital Tuesday as an intense shelling attack targeted the country’s second-largest city, and both sides looked to resume talks in the coming days aimed at stopping the fighting.

The country’s embattled president said he believed the stepped-up shelling was designed to force him into concessions.

“I believe Russia is trying to put pressure (on Ukraine) with this simple method,” Ukrainian President Volodymyr Zelenskyy said late Monday in a video address. He did not offer details of hours-long talks that took place Monday, but he said Kyiv was not prepared to make concessions “when one side is hitting another with rocket artillery.”

The developments came as Russia finds itself increasingly isolated as a result of international condemnation and potentially backbreaking economic sanctions. Five days into the invasion, the Russian military’s movements have been stalled by fierce resistance on the ground and a surprising inability to dominate the airspace.

The Kremlin has twice in as many days raised the spectre of nuclear war and put on high alert an arsenal including intercontinental ballistic missiles and long-range bombers. Stepping up his rhetoric, President Vladimir Putin denounced the U.S. and its allies as an “empire of lies.”