Washington (AFP) - Pro-Russian rebels in eastern Ukraine have broken a days-old ceasefire more than 250 times since it went into effect earlier this week, a US official said Thursday.

"The United States condemns continuing attacks by Russia-backed separatists in and around Debaltseve, Mariupol and other locations in eastern Ukraine which violate the ceasefire and flout the Minsk agreements," State Department spokeswoman Jen Psaki told reporters.

The leaders of Ukraine, Germany, France and Russia on Thursday pledged renewed support for the tattered ceasefire in eastern Ukraine despite the violations.

The hard-won truce went into effect in the pre-dawn hours of Sunday morning in Ukraine.

But Psaki said the Ukrainian government had accused the separatists of breaking the ceasefire "more than 250 times" since it began on February 15.

The rebels have also seized the key town of Debaltseve -- a strategic railway hub -- in a major blow to Kiev's military forces.

Psaki repeated US calls on Russia and the separatists "to stop their attacks immediately, withdraw heavy weapons, halt the flow of fighters and equipment from Russia into Ukraine."

She also urged that OSCE monitors be allowed into the areas under rebel control to verify whether the ceasefire was holding.

The United States was not around the negotiating table for the talks with Germany, France, Russia and Ukraine which hammered out the peace deal.

US Secretary of State John Kerry is however flying to London this weekend for talks with his British counterpart Philip Hammond on Saturday "to discuss bilateral and global issues" with the Iran nuclear negotiations as well as the crisis in Ukraine likely to top the agenda.