A Downing Street spokesman said:

The Prime Minister spoke to President Obama this evening, following today’s Contact Group meeting in Geneva.

They agreed that the outcome of the Geneva meeting represented a positive step forward, but that it was essential that the agreements reached were rapidly implemented in order to reduce tensions and create space for political dialogue in Ukraine.

They agreed that the United States and European Union should do all they could to help implement these agreements, in particular through providing additional support to the OSCE Special Monitoring Mission.

The United Kingdom will provide an additional £1 million to support the expansion of the mission and help it deliver its mandate. The Prime Minister and President agreed that in the meantime the EU and US should continue preparatory work on potential additional sanctions, so that we are ready to respond quickly if the agreement were not implemented.

https://www.gov.uk/government/news/call-with-president-obama