US President Barack Obama and German Chancellor Angela Merkel said Sunday that Western sanctions must remain in place against Russia until it complies with a ceasefire deal with Ukraine.

"The two leaders discussed the ongoing crisis in Ukraine and agreed that the duration of sanctions should be clearly linked to Russia's full implementation of the Minsk agreements and respect for Ukraine's sovereignty," the White House said in a statement released on the sidelines of the G7 summit in Germany.

Obama and Merkel held private talks ahead of the summit and the US president earlier urged his fellow G7 leaders to stand up together against "Russian aggression in Ukraine".

Peace talks on Ukraine resumed in Minsk on May 6 as clashes have intensified on the ground in a conflict that has killed more than 6,400 people since April 2014.

The negotiations are aimed at enforcing the implementation of a peace deal reached in Minsk in February, leading to an extremely fragile ceasefire that is regularly breached across the eastern conflict zone.

The Minsk deal, agreed under pressure from the leaders of Germany and France, states that all conditions -- including a complete ceasefire, the pullback of all heavy weapons and the restoration of Ukraine's border with Russia to Ukrainian control -- should be fulfilled by the end of 2015.

The United States, the EU and Germany all expressed alarm over the renewed violence in eastern Ukraine, with Washington warning Russia it would face new punishment if the crisis escalates.

Diplomats told AFP Friday that EU leaders meeting in Brussels this month were set to extend sanctions on Russia over its involvement in the fighting in Ukraine until the end of 2015.

The sanctions, which target economic sectors such as Russia's banks and oil industry, were imposed after the shooting down of Malaysia Airlines flight MH17 over rebel-held eastern Ukraine in July 2014.

The White House said Obama and Merkel also discussed the proposed TTIP transatlantic trade deal "to promote growth and jobs on both sides of the Atlantic" as well as "the importance of working together to promote a successful climate agreement this year".

Global leaders will gather at a year-end United Nations summit in Paris in a bid to forge a new treaty aimed at fighting global warming.

Merkel has said she hopes the G7 summit, running until Monday, will lend momentum to the drive for a Paris accord.

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