The call came on the eve of a UN meeting Thursday to discuss the crisis on Ukraine's borders.
British Prime Minister Boris Johnson said Wednesday there was "currently little evidence of Russia disengaging" from its military buildup near Ukraine, in a phone call with United Nations Secretary-General Antonio Guterres.
"The leaders reiterated states' responsibility to abide by their obligations under the United Nations Charter and to respect the sovereignty and territorial integrity of UN members," Johnson's office said in a readout of the call.
The pair also agreed that any Russian invasion of Ukraine would have "catastrophic and far-reaching consequences", a Downing Street spokeswoman added.
The call came on the eve of a UN Security Council meeting Thursday to discuss the crisis on Ukraine's borders, where Moscow has massed more than 100,000 soldiers.
Britain will be represented at the meeting by its Europe Minister James Cleverly, Downing Street noted.
Johnson and Guterres "reaffirmed the importance of all parties working in good faith" to implement existing agreements struck by Ukraine, Russia and mediating countries concerning the region's security, it added.
"They agreed to continue working closely together to pursue an urgent diplomatic resolution and avert a disastrous military escalation and humanitarian crisis," the Downing Street spokeswoman said.
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