By Natalia Zinets
KIEV, June 16 (Reuters) - Ukraine's president said on Monday he had ordered troops to regain control of the frontier with Russia to pave the way for a truce and peace talks after weeks of fighting with pro-Russian separatists.
President Petro Poroshenko did not say how long the truce should last in Ukraine's east, where the rebels have risen up against central rule, but he said it could start only if the long and porous border was secure.
His remarks underlined his concern that Russia is supporting the rebels by sending tanks, guns and fighters across the border, especially after the separatists shot down a military transport plane on Saturday. Russia denies the accusation.
"The ceasefire will be declared as soon as the border is secure," Poroshenko told a meeting of his top security chiefs in Kiev, setting his sights on later this week. "Declaring a ceasefire while the border is open would be irresponsible."
Poroshenko called the meeting after promising a resolute response to the shooting down of the plane on Saturday near the eastern city of Luhansk, which killed 49 servicemen on board.
But his comments indicated that, rather than hit the rebels with the full might of his army, he would continue a two-pronged policy of trying to reach a peace deal and allowing civilians to be evacuated, while pressing on with a military campaign.
The initial rebel response was dismissive.
"No one believes a word Poroshenko says here. He said there would be a humanitarian corridor for women and children and half an hour later they were shelling (the city of) Slaviansk," said a spokesman for the self-proclaimed Donetsk People's Republic.
He described it as a ceasefire "just for us and they'll stay on the offensive. Let's see if they stop shooting first.'
The separatists rose up against rule from Kiev in April following the removal of a president who was sympathetic to Moscow, and Russia's subsequent annexation of the Black Sea peninsula of Crimea.
Scores of government troops, rebels and civilians have been killed in fighting since then but unleashing the armed forces' full strength might antagonize President Vladimir Putin and increase pressure on him in Russia to invade east Ukraine.
Since he was sworn in on June 7, Poroshenko has been trying to win agreement for his peace proposals while intensifying the "Anti-Terrorist Operation" to prise the rebels out of several towns and cities they hold in the Russian-speaking east.
Poroshenko said his proposals included constitutional changes to allow more decentralization of power, a demand made by many people in eastern Ukraine.
"There are key elements on decentralizing power in the peace plan ... There are constitutional changes," he said, also holding out the possibility of an amnesty for fighters who lay down their arms and do not have blood on their hands.
He said government forces had already re-established control of a more than 250-km (156-mile) stretch of the about 2,000-km land border with Russia. (Additional reporting by Alessandra Prentice in Donetsk; Writing by Timothy Heritage; Editing by Catherine Evans)