KYIV, Ukraine, March 6 (Reuters) - The number of refugees fleeing Ukraine was expected to reach 1.5 million on Sunday as Russia continued its attack 11 days after invading Ukraine and Kyiv pressed for further Western help, including more sanctions and weapons.

Moscow and Kyiv traded blame over a failed ceasefire on Saturday that would have let civilians flee Mariupol and Volnovakha, two southern cities besieged by Russian forces. Ukrainians who could escape spilled into neighbouring Poland, Romania, Slovakia and elsewhere.

Ukrainian negotiators said a third round of talks with Russia on a ceasefire would go ahead on Monday, although Moscow was less definitive.

In a televised address on Saturday night, Ukraine's President Volodymyr Zelenskiy called on people in areas occupied by Russian troops to fight.

"We must go outside and drive this evil out of our cities," he said, vowing to rebuild his nation.

Russian President Vladimir Putin reiterated that he wanted a neutral Ukraine that had been "demilitarised" and "denazified," and likened Western sanctions "to a declaration of war," adding: "Thank God it has not come to that."

Ukraine and Western countries have decried Putin's reasons as a baseless pretext for the invasion and have imposed sweeping sanctions aimed at isolating Moscow and crippling its economy.