Mr. Poroshenko laid out the broad outlines of the peace plan during his inauguration earlier this month, saying he was willing to negotiate and offer an amnesty to fighters who didn't have "blood on their hands."
This softer approach backtracked from his call following his election victory in May for an accelerated military operation to stamp out the pro-Russian rebellion.
But the president said that a cease fire could only happen after Ukraine regains full control of its border with Russia, of which the separatists now control wide stretches in the Donetsk and Luhansk regions.
Ukraine has accused Russia of allowing arms and fighters to cross the boundary unhindered and last week Kiev and the U.S. said there was evidence the Kremlin had armed fighters with tanks and heavy weapons.
"As soon as the border is secured, we shall be able to declare a cease fire straight away. Declaring a cease fire now with an open border is irresponsible given the situation that exists in Donetsk and Luhansk," Mr. Poroshenko said before a meeting of Ukraine's National Security and Defense Council.
He said the length of the cease fire would be limited and the immediate goal would be to guarantee the safety of people living in the separatist-controlled regions and to stop looting there.
After that, the focus would be on disarmament and sorting out an amnesty for those who haven't committed "egregious crimes."
The council's secretary, Andriy Parubiy, earlier in the day said there are now up to 20,000 separatist fighters in the region and that more than half of them had come from Russia.
He also said there were signs that Russian soldiers—who Russian President Vladimir Putin had ordered back to their bases from the border last month—were returning to their positions along the frontier.
A military spokesman for the North Atlantic Treaty Organization said while there are signs of Russian troop movement on a regular basis, there was no evidence yet of a substantial build up on the border.
Russia had stationed an estimated 40,000 soldiers along the border following the ouster of Ukraine's then President Viktor Yanukovych in February, but NATO says the vast majority appear to have pulled back.
The Organization for Security and Cooperation in Europe said Monday that fighting in eastern Ukraine has damaged a key water pumping station outside the regional capital of Donetsk, threatening water supplies to four million people.
The Donetsk region has been the scene of some of the heaviest fighting between pro-Russian separatists and Kiev government forces, and the city of Donetsk is now firmly in rebel hands.
Last week, Kiev moved the regional administration from Donetsk to the city of Mariupol.
The disruption of water supplies raises the possibility of a humanitarian crisis.
Last week, Mr. Poroshenko said an escape corridor should be created.
Ukraine's military—whose advance into rebel-controlled areas has been slowed by the presence of civilians—has said it has been allowing people to leave conflict zones.
The OSCE said local officials told them that recent fighting near Semyonivka, about 3 miles southeast of the rebel stronghold of Slavyansk, had knocked out the pumping station and a section of pipeline that runs along the main road leading to the city of Donetsk.
The pipeline and pumping station serve as the main water supply for the 1 million people who live in Donetsk and another three million people who inhabit the region, the organization said.
In a statement, the OSCE quoted the city's elected mayor, Alexander Lukyanchenko as saying that water supplies to the city had not yet been affected, but would be "in a very short while."
He said efforts to repair the damage had begun but were stopped due to renewed fighting in the area.
The OSCE said it was attempting to contact both sides in the fighting to ensure the repair work could resume.
Meanwhile, officials at the self-declared Donetsk People's Republic briefly took control of the regional offices of Ukraine's National Bank and the state treasury.
One separatist official was quoted by the Interfax news agency as saying the seizure was done to stop any money or tax revenues from the region from going to the central government.
In a statement, the Kiev government said the seizure paralyzed Ukraine's financial system in the region, making it impossible to pay pensions, government wages, social benefits or subsidies to local industries.
But Mr. Lukyanchenko later said following negotiations, the separatists agreed to surrender the central bank's offices and allow the payments to continue so as not to further disrupt peoples' lives.
Source: The Wall Street Journal