Joe Biden’s ambassador to the United Nations warned on Sunday there was little immediate hope of a negotiated end to the war in Ukraine, as pressure continued to build on the US president ahead of a crucial Nato summit in Europe this week.

Linda Thomas-Greenfield was reacting on CNN’s State of the Union to an interview with Volodymr Zelenskiy in which the Ukrainian president told the same network only talks would end the war and its devastating toll on civilians.

“We have to use any format, any chance, to have the possibility of negotiating, of talking to [Russian president Vladimir] Putin,” Zelenskiy told Fareed Zakaria, the host of GPS. “If these attempts fail, that would mean that this is a third world war.”

Thomas-Greenfield said she saw little chance of a breakthrough.

“We have supported the negotiations that President Zelenskiy has attempted with the Russians, and I use the word attempted because the negotiations seem to be one-sided, and the Russians have not leaned in to any possibility for a negotiated and diplomatic solution,” she said.

“We tried before Russia decided to move forward in this brutal attack on Ukraine and those diplomatic efforts were not responded to well by the Russians, and they’re not responding now. But we’re still hopeful that the Ukrainian effort will end this brutal war.”

The Nato secretary general, Jens Stoltenberg, told NBC’s Meet the Press: “Turkey is doing some real effort to try to facilitate, support talks between Russia and Ukraine. It’s far too early to say whether these talks can lead to any concrete outcome.”

Biden, who faces growing dissatisfaction over his approach to the war, will travel to Brussels on Thursday. He will hear a proposal from Poland for Nato to send a peacekeeping force into Ukraine, something Thomas-Greenfield said was unlikely.

“I can’t preview what decisions will be made and how Nato will respond to the Polish proposal,” she said. “What I can say is American troops will not be on the ground in Ukraine at this moment. The president has been clear on that.

“Other Nato countries may decide that they want to put troops inside of Ukraine, that will be a decision that they have made. We don’t want to escalate this into a war with the United States but we will support our Nato allies.”

Thomas-Greenfield was asked about reports that thousands of residents of the besieged city of Mariupol have been deported to Russia.

“I’ve only heard it,” she said. “I can’t confirm it. But I can say it is disturbing. It is unconscionable for Russia to force Ukrainian citizens into Russia and put them in what will basically be concentration and prisoner camps.”

Republicans were critical of the pace and content of US support for Ukraine. Following Zelenskiy’s address to Congress on Wednesday, the White House announced $800m in military aid, following a $13.6bn package. But Biden has rejected a no-fly zone and the transfer of Polish Mig fighter jets.

“The president has had to be pushed and pulled to where he is today,” the Wyoming Republican senator John Barasso told ABC’s This Week.

“It was Congress that brought about sanctions, that brought about the ban on Russian oil, that brought about weapons and all of this big aid package. So far the administration has only released $1bn of that. We might not have been in this situation if they had done punishing sanctions before the tanks began to roll.”

Speaking to CBS’s Face the Nation, the Senate minority leader, Mitch McConnell, said he believed Biden “needs to step up his game”.

The president, McConnell said: “has generally done the right thing but never soon enough. I am perplexed as to why we couldn’t get the Polish-Russian Migs into the country.”

McConnell added that Biden should visit friendly countries close to the conflict zone, such as Romania, Poland, and the Baltic nations.

“They’re right on the frontlines and need to know that we’re in this fight with them to win,” he said.

McConnell also condemned Republican extremists who have opposed support for Ukraine, such as the North Carolina congressman Madison Cawthorne, who has called Zelenskiy “a thug”.

“There are some lonely voices out there who are in a different place,” McConnell said.

Concern is rising among Biden’s allies. Dick Durbin of Illinois, the Democratic Senate whip, reiterated the call to approve air support for Ukraine.

“We’re asking for one-third of the Polish air force to be sent into Ukraine,” he told ABC. The people of Poland, of course, want to make certain that they’re safe. They’re only a few miles away from the devastation that’s going on in Ukraine.

“There are other ways for us to provide surface-to-air missiles and air defenses that will keep the Russians at bay in terms of their aerial attacks. There are ways to do that that are consistent with the Nato alliance and would not jeopardise expanding this into world war three or even worse.”

Marek Magierowski, the Polish ambassador to the US, stressed that the proposal for a peacekeeping force in Ukraine was only “a preliminary concept”.

“We can’t take any decisions unilaterally, they have to be taken by all Nato members,” he told CNN, adding: “If there is an incursion into Nato territory, I believe that Russia can expect a very harsh response on the part of our alliance.”

Zelenskiy lamented the provision only of economic and limited military support.

“If we were a Nato member, a war wouldn’t have started,” he said. “If Nato members are ready to see us in the alliance, do it immediately because people are dying on a daily basis.

“But if you are not ready to preserve the lives of our people, if you just want to see us straddle two worlds, if you want to see us in this dubious position where we don’t understand whether you can accept us or not, you cannot place us in this situation, you cannot force us to be in this limbo.”

Zelenskiy, however, appeared to acknowledge last week that Ukraine would not join Nato.

On CBS’s Face the Nation, the US defense secretary, Lloyd Austin, said the use of chemical weapons by Russia, which many analysts predict, would produce a “significant reaction” from the US and the international community.

On NBC, Stoltenberg said the use of chemical weapons “would be a blatant and brutal violation of international law”. But he would not say such an outcome would change Nato policy towards intervention.

Biden this week spoke to the Chinese president, Xi Jinping, seeking to prevent support for Russia. The Chinese ambassador to the US, Qin Gang, spoke to CBS.

He said: “What China is doing is sending food, medicine, sleeping bags and baby formula, not weapons and ammunition to any party.”

Gang also said Chinese condemnation of the Russian invasion, for which some have called, would not “solve the problem”.

“I would be surprised if Russia will back down by condemnation,” he said.

In Ukraine, fighting continues. The retired US army general and former CIA director David Petraeus told CNN the conflict had reached “a bloody stalemate, with lots of continued damage on both sides, lots of destruction, especially from the Russians”.