* Ukrainian leader says work to start on truce 'roadmap'
* Putin says not Russia's role to negotiate ceasefire terms
* Summit host Belarus says fundamental differences remain (Updates after presidents' statements)
By Alexei Anishchuk and Natalia Zinets
MINSK, Aug 27 (Reuters) - Ukrainian President Petro Poroshenko promised after late-night talks with Russia's Vladimir Putin to work on an urgent ceasefire plan to defuse the separatist conflict in the east of his former Soviet republic.
The first negotiations between the two leaders since June were described by Putin as positive, but he said it was not for Russia to get into the details of truce terms between the Kiev government and two rebel eastern regions.
"We didn't substantively discuss that, and we, Russia, can't substantively discuss conditions of a ceasefire, of agreements between Kiev, Donetsk and Luhansk. That's not our business, it's up to Ukraine itself," he told reporters early on Wednesday.
"We can only contribute to create a situation of trust for a possible, and in my view, extremely necessary, negotiation process."
Poroshenko, after two hours of one-to-one talks which he described as "very tough and complex", told reporters: "A roadmap will be prepared in order to achieve as soon as possible a ceasefire regime which absolutely must be bilateral in character."
Despite the positive tone, it remained unclear how the rebels would respond to the idea of a ceasefire, how soon it could be agreed and how long it might stick.
And with Putin insisting the details were an internal matter for Kiev, there was no sign of progress on a fundamental point of disagreement: Ukraine's charges that Moscow is sending arms and fighters to help the rebels, and Russia's adamant denials.
The leaders shook hands at the start of their meeting in the Belarussian capital Minsk just hours after Kiev said it had captured Russian soldiers on a "special mission" on Ukrainian territory.
Responding to a video of the detained servicemen, a Russian defence ministry source told Russian news agencies that they had crossed the border by mistake. But Ukraine's military spokesman dismissed that, mocking the idea that "the paratroopers got lost like Little Red Riding Hood in the forest".
The Minsk talks, preceded by six hours of wider negotiations with top European Union officials and the presidents of Belarus and Kazakhstan, were aimed at ending five months of conflict that has heightened tensions between Russia and NATO.
A United Nations report obtained by Reuters said more than 2,200 people have been killed. The crisis has prompted the United States and EU to slap sanctions on Russia, drawing retaliation from Moscow in a trade battle that is hurting the Russian and European economies.
"We all wanted a breakthrough," President Alexander Lukashenko of Belarus told reporters after the multilateral part of the talks had finished.
"But the very fact of holding the meeting today is already a success, undoubtedly," he said. "The talks were difficult. The sides' positions differ, sometimes fundamentally ... Everybody agreed on the need to de-escalate and free hostages."
EU foreign policy chief Catherine Ashton told reporters: "It was cordial but positive. There was a sense in which the onus was on everyone to see if they could do their best to try to resolve this."
In a televised statement at the start of the talks, Putin urged Poroshenko not to step up his offensive against the pro-Moscow rebels, and threatened to slap economic penalties on Kiev for signing a trade accord with the European Union that he said would squeeze Russian goods out of its market.
Poroshenko replied by demanding a halt to arms shipments from Russia to the separatist fighters.
In his comments after the meetings, Putin said the two presidents agreed to talks on a resumption of gas supplies to Ukraine, which Russia severed in mid-June in a dispute over pricing and debt.
"We need to resume our energy dialogue, including about gas problems. Sincerely speaking, this is a complicated issue, it has reached a dead end, but we still need to talk about it. We agreed to resume these consultations," Putin said. (Additional reporting by Andrei Makhovsky, Alessandra Prentice, Richard Balmforth, Pavel Polityuk, Thomas Grove and Anton Zverev; Writing by Mark Trevelyan; Editing by Bernard Orr)