Biden pointedly drove home the fundamental U.S. retaliatory plan to Russia's invasion - Washington was prepared to do everything to defend NATO from attack, but would not risk triggering “World War Three” by engaging directly with Russia.

Ukrainian Foreign Minister Dmytro Kuleba met his Russian opposite number Sergei Lavrov in Turkey but an hour of talks produced no results.

Russian Foreign Minister Sergei Lavrov was due to meet his Ukrainian opposite number Dmytro Kuleba in Turkey – the first high-level diplomacy undertaken between the two countries since Moscow launched the invasion of its neighbour on 24th February.

But continued shelling prevented a planned evacuation of the port of Mariupol on the Sea of Azov, where food and water were in very short supply after a week of Russian encirclement.

Ukraine flatly rejected as “immoral” a Russian proposal to allow residents of the towns --  including Mariupol on the Sea of Azov, Sumy in the north, the capital Kyiv and Ukraine’s second city Kharkiv -- that would have sent them into the territory of Russia or Belarus, Moscow’s close ally.

The Russian military relentlessly pressed on with its drive to get Ukrainian cities to submit, shelling several towns already largely in ruins. But in the sole city seized by Russian forces, Kherson in Ukraine’s south, dozens of demonstrators staged a demonstration, showing disregard for the occupiers.

Russia’s pressed on with its drive to shell Ukrainian cities into submission, taking a stranglehold on the southern port of Mariupol and taking aim at Mykolayiv and Odesa further west

Ukrainian President Volodymyr Zelensky called for an urgent meeting with Kremlin leader Vladimir Putin to stop the fighting that has engulfed much of the country and sent more than a million Ukrainians fleeing across international borders. He repeated calls for a Western-installed “no fly zone” over Ukraine and, failing that, asked for aircraft to bolster his country’s defences.