The four meet in Berlin from 1745 GMT to assess progress in the Minsk truce deal agreed in February between the Ukraine government and pro-Russian rebels to end a conflict that has killed more than 6,000 people over the past year.
At stake in the longterm for Moscow is the easing of punishing Western sanctions introduced as separatist insurgents allegedly operating with Russian military assistance seized control of a swathe of territory in the east.
"The situation remains fragile," he added, "but it is worth continuing, which is why we need to see Kiev and Moscow ready to work."
NATO however believes Russia has supplied more troops and weapons to the rebels, an unnamed alliance official was quoted as saying at the weekend by German paper, Frankfurter Allgemeine Zeitung.
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After weeks in which the ceasefire agreed in the Belarussian capital appeared to be largely holding despite isolated skirmishes, clashes seemed to be mounting in flashpoint areas.
In accordance with the Minsk deal the two sides claim to have withdrawn heavy arms from the frontline but the OSCE said "weapons with a calibre larger than 100mm were used by both sides during the fighting".
Around Shyrokyne, near the strategic port of Mariupol, the largest remaining city in the conflict zone still in government hands, the OSCE said dozens of mortar rounds were traded yesterday and heavy artillery fire from rebels "shook buildings across Mariupol".