JOHN YANG: Good evening, I'm John Yang.

The Israeli military says it will limit fighting in a small area of southern Gaza in order to allow more aid to reach Gazans desperately in need of food.

Daytime military operations will pause along a seven mile stretch of road between the Kerem Shalom crossing with Israel and a hospital near Khan Younis.

The pace of aid shipments destined for Gaza slowed when Israel closed the Rafah crossing with Egypt.

Far-right ultranationalist and Prime Minister Benjamin Netanyahu's coalition government quickly condemned the move.

All of this unfolded as world leaders gathered in Switzerland for a Ukraine peace summit where White House National Security Adviser Jake Sullivan played a key role.

He sat down one on one with foreign affairs and defense correspondent Nick Schifrin.

NICK SCHIFRIN: I want to get to Ukraine and the peace summit.

But let me just ask you first to respond to an Israeli Defense Forces announcement this morning of what they're calling, quote, a tactical humanitarian pause from 8:00 a.m. to 7:00 p.m. every day leading from Kerem Shalom, the crossing between Israel into Gaza, going toward Rafah, how significant is that announcement?

JAKE SULLIVAN, National Security Advisor: It's significant.

This is a good and important step, because we've made a huge amount of progress in getting aid into Gaza, including the work that President Biden did to get Kerem Shalom back open a few weeks ago after the rough operation began.

But the issue now has been getting aid around Gaza so that people can get access to life saving food, and medicine and water.

And what this tactical pause will allow for is the free flow of humanitarian trucks without a concern that they could be targeted or delayed because of military operations.

That's going to help the people of Gaza and this is something that the Israeli Defense Forces deserve credit for.

And the U.N. deserves credit for it as well, because it was through a consultation between the U.N. and the IDF that this came to pass with the support in the United States.

NICK SCHIFRIN: The U.N. blames Israel for blocking some of the movement of trucks.

And there's still a shortage of vast shortage of trucks inside of Gaza.

This doesn't change that, does that?

JAKE SULLIVAN: No, the issue of just the number of trucks has remained a persistent problem, one that we have worked to try to solve by increasing the flow, not just the flow in and out of trucks, but the overall stock of trucks that can move goods around Gaza.

We've made some progress in that regard working with the U.N. who contracts for new trucks to be brought in.

There's more progress that's required.

And of course, when you're in a war zone like this, there's always going to be difficulty in the distribution of humanitarian aid.

And that's why we have worked so hard to encourage the Israelis to take every measure possible, including humanitarian pauses to help make the life as easy as possible for humanitarian organs.

NICK SCHIFRIN: But I assume you are still calling for the idea for the Israelis to take as many steps as they can to alleviate some of these channels.

JAKE SULLIVAN: Of course we are and even beyond that, and this is a critical point.

We are not going to get to a fully satisfactory humanitarian situation until there is a ceasefire.

That's why President Biden has called for a ceasefire and hostage deal.

He'd like to see an end to the hostilities and end to the war, because it's only in that circumstance, that the people of Gaza can be fully safe and have full and complete access to the humanitarian needs that that they so badly have, you know, been struggling to get over the course of the past several months.

NICK SCHIFRIN: Let's move here to Switzerland and Ukraine.

What can you accomplish at a peace summit that Russia is not attending, nor is China that has helped rebuild Russia's defense industrial base, and countries like Saudi Arabia and Kenya, are calling on Ukraine and Russia to negotiate directly?

JAKE SULLIVAN: Well, first, Ukraine has made clear President Zelenskyy has made clear including here in Switzerland that ultimately, this war will have to be determined at the negotiating table.

The United States, his goal has been to put Ukraine in as strong as possible a position on the battlefield.

So it is as in as strong a position as possible at the negotiating table.

But its strength at the negotiating table will also be improved by what's happened here in Switzerland, because you have more than 100 countries and international organizations coming together to lay a foundation for a just peace built on a simple proposition.

You cannot take territory by force.

It says that in the United Nations Charter, it's at the core of international law, Ukraine's sovereignty and territorial integrity has to be respected.

And the fact that you have a broad chorus of countries, not just from Europe, not just the United States, but from around the world saying this, that is going to strengthen Ukraine's hand when it eventually negotiates with Russia.

And that's up to Ukraine to decide when and how that occurs.

NICK SCHIFRIN: You said this weekend that China is not here, because it quote, obliged Putin, who's calling the shots and the so called No Limits relationship?

JAKE SULLIVAN: Well, look, each country makes its own decision.

But it certainly looks to us like Russia went to China and said, please don't go and China obliged.

We think that that's not a positive sign for China's commitment to adjust peace in Ukraine, because Russia only gets more and more extreme in its demands, including what President Putin said just recently.

And if you look at the range of countries present here, and the basic outcome of this summit, which is a reaffirmation of U.N. Charter principles that China itself signed up to China should be here and China should be supporting a just peace based on those principles.

And based on international law, the fact that it's siding with Russia on this issue, I think is not a positive development.

JOHN YANG: More of next conversation with Jake Sullivan will be on tomorrow's PBS NewsHour.