GWEN IFILL: Ukrainian president Petro Poroshenko told his military today to prepare for a possible full-scale invasion from Russia. The remarks came one day after a new outbreak of fighting in Eastern Ukraine, which claimed the lives of more than 25 people and injured dozens more.

Speaking to the Ukrainian Parliament, Poroshenko described what he called a colossal threat of the resumption of large-scale hostilities by Russian and terrorist forces. And he said, contrary to Moscow's denials, those Russian troops are in rebel-controlled areas.

PRESIDENT PETRO POROSHENKO, Ukraine (through interpreter): In Ukrainian territory, there are 14 Russian battalions, tactical groups that include over 9,000 Russian military servicemen. The concentration of Russian servicemen along the national border with Ukraine is 1.5 times bigger than a year ago.

The military must be ready as much for a renewal of an offensive by the enemy in the Donbass as they are for a full-scale invasion along the whole length of the border with Russia. We must be truly ready for this.

GWEN IFILL: A cease-fire signed in February has been steadily crumbling, as international monitors report regular violations.

David Herszenhorn of The New York Times has been following the story from Moscow.

Those were pretty strong words from Petro Poroshenko today, David. And they were backed up from what NATO — by what NATO has been saying as well.

DAVID HERSZENHORN, The New York Times: There's no question, Gwen.

What we see is that we have entered a very tense, in fact, obviously quite dangerous period of time, where Ukraine's Western allies, especially France and Germany, have become extremely frustrated by the slow pace, the failure actually to implement that Minsk 2 cease-fire accord signed back in February.

And so what we see is an increase in hostilities, as both sides are trying to show that the other is responsible for these violations, for the increase in violence and for the failure to move forward with political compromises need for a long-term settlement.

GWEN IFILL: What has happened between — since February, between now and then, that's resulted in this latest kind of explosion of violence?

DAVID HERSZENHORN: Well, what we see now is that the European Union has begun to consider whether to extend economic sanctions against Russia. Those are set to expire without any further action by the end of July.

The cease-fire had been mostly holding, but as each side tries to position itself, of course, we expect that this kind of violence will flare up. At the same time, Petro Poroshenko has now come upon a full year since his inauguration. It's been a very tough year for Ukraine. He made that clear in his speech today.

The country is suffering greatly. And the economy is on the verge of collapse. They feel this constant threat of war. And so the tensions really are running high. And now, as we push toward this question of a renewal of sanctions, each side wants to show that the other is more at fault, laying the groundwork for what could be tough talks ahead.

GWEN IFILL: What, if any, evidence does Poroshenko or NATO or anyone have of the presence of Russian activity along the border?

DAVID HERSZENHORN: Well, there is no question, amid all the finger-pointing, there are certain things that we know in fact are true.

There's a lot of heavy equipment, tanks, weaponry that is on the ground still in Eastern Ukraine. Most of that appears, if not all of it, to have come from Russia across the border, which is unsecured. That border is open. There are vehicles, fighters able to pass back and forth, whether they're volunteers, as Russia has insisted, or regular forces of the Russian Federation, as Mr. Poroshenko is suggesting.

So we have that evidence. We also have the international monitors from the Organization for Security and Cooperation in Europe pointing out the other day that they'd actually encountered some fighters in Russian uniform. They have not said that before, where they actually see the insignias of the Russian military.

At the same time, Ukraine has captured two soldiers, fighters that it said were regular duty active Russian military personnel. They have said the same thing. Of course, they're in captivity. They're being held in a military hospital in Kiev, the hospital, and so there is increasing evidence that in fact Russia's intervention in Ukraine continues.

GWEN IFILL: So does that mean that the cease-fire that was so painstakingly hammered out is basically no longer worth the paper it's printed on?

DAVID HERSZENHORN: Well, the cease-fire has been precarious all along.

What we have seen now is that certainly Ukraine will be pressuring its allies, the United States and Europe, to make sure that there is no letup in economic sanctions against Russia. There is obviously fatigue in the West. Some of the European countries are suffering quite a great deal themselves from countersanctions that Russia has imposed. So I think we will see again quite a bit of tension as we move toward this question in Brussels of whether to renew sanctions or not.

GWEN IFILL: So, no matter how you look at it, whether the sanctions are working or not, whether Russia is there or not, it sounds like there is another spiral under way.

DAVID HERSZENHORN: There's no question there's another spiral under way.

And what the West is grappling with is really a lack of will on both sides to implement that cease-fire accord that was signed in February. For each side, it seems that the status quo is preferable to moving forward with the really tough political decisions that need to be made.

One of the things Mr. Poroshenko has been pushing for is a decentralization process in Ukraine, which would give more power to mayors, essentially making governors and officials at the regional level sort of an intermediary between the federal government and local government. Of course, in Eastern Ukraine, what the separatists, pro-Russian side, and Russia would like to see, are very strong governors, perhaps even presidents of the autonomous regions of Donetsk and Luhansk, who are potentially more loyal to Moscow than they are to Kiev.

So there are very different views of what the future politically ought to look like in Ukraine. And pushing off those tough questions seems to be what a lot of these new hostilities are about.

GWEN IFILL: David Herszenhorn, reporting from Moscow for The New York Times tonight, thank you.

DAVID HERSZENHORN: Thank you.