This week will be "very critical" for Russia-Ukraine war talks as Washington decides if it is an "endeavour that we want to continue to be involved in", US Secretary of State Marco Rubio said.

Rubio told US media that Donald Trump hasn't imposed new penalties on Russia because he still hopes diplomacy can end the war.

It comes after Trump held a brief meeting with Ukrainian leader Volodymyr Zelensky at the Vatican before Pope Francis' funeral on Saturday.

Updating reporters on Sunday, Trump said he believes Zelensky is willing to give up Crimea to Russia as part of a peace deal - despite Kyiv's previous rejections of any such proposal.

Speaking to NBC's Meet the Press programme, Rubio said: "There are reasons to be optimistic, but there are reasons to be realistic.

"We're close, but we're not close enough. Throughout this process, it's about determining, do both sides really want peace and how close are they or how far apart they are after 90 days of effort here... that's what we're trying to determine this week."

Addressing the prospect of imposing penalties on Vladimir Putin, Rubio said: "The minute you start doing that kind of stuff, you're walking away from it."

Over the weekend Trump questioned whether the Russian president wants to "stop the war" against Ukraine.

"It makes me think that maybe he doesn't want to stop the war, he's just tapping me along, and has to be dealt with differently, through "Banking" or "Secondary Sanctions?" Too many people are dying!!!" Trump wrote on Truth Social.

His post came just after he held a private meeting with Zelensky in Italy, which the White House described as a "productive discussion."

Asked on Sunday if he thought the Ukrainian president was ready to cede control of its southern peninsula, which was illegally annexed by Russia in 2014, Trump replied: "I think so."

Trump also said Zelensky seemed "calmer", in what could have been a reference to a very public clash between the two presidents at the White House in February.

Ukraine has repeatedly rejected making any territorial concessions, stressing that issues about land should only be discussed once a ceasefire is agreed.

Neither Zelensky nor Putin have publicly responded to Trump's latest comments.

Elsewhere on Sunday, German Defence Minister Boris Pistorius warned Ukraine not to agree to a deal which involves sweeping territorial concessions in return for a ceasefire.

He told German public broadcaster ARD that Kyiv "should not go as far as the latest proposal by the American president", which he said would amount to a "capitulation".

The German minister said Ukraine knew it might have to part with some territory to secure a truce.

"But they will certainly not go as far - or should not go as far - as the latest proposal by the American president.

"Ukraine could have got a year ago what was included in that proposal, it is akin to a capitulation. I cannot discern any added value," Pistorius said.

Trump said last week that "most of the major points [of the deal] are agreed to". Reports suggest that Ukraine could be asked to give up large portions of land seized by Russia, including Crimea.

The BBC has not seen the exact details of the latest US plan.

On Friday, Reuters news agency reported that it had seen proposals from the US that included American legal acceptance of Russia's illegal annexation of Crimea and de facto recognition of Russian control of other occupied areas, including all of Luhansk in the east of the country.

Reuters says it has also seen counter-proposals from Europe and Ukraine, which reportedly say the sides will only discuss what happens to occupied Ukrainian territory once a ceasefire has come into effect.

The US plan also rules out Ukraine's membership in the Nato military alliance and sees a UK-France led "coalition of the willing" providing a security guarantee once a ceasefire is in force without the involvement of the US.

Meanwhile Europe want the US to give "robust" guarantees in the form of a cast-iron Nato-style commitment to come to Ukraine's aid if it is attacked.

The US reportedly further proposes to take control of the Zaporizhzhia nuclear plant - currently occupied by Russia - which would then provide electricity to both Russia and Ukraine. The counter-plan makes no mention of giving Russia power.

In an interview with Time magazine this week, Trump once again blamed Kyiv for starting the war, citing its ambitions of joining Nato.

The US president also told Time: "Crimea will stay with Russia."

The US has warned it would walk away from negotiations if progress was not made.

Russia launched a full-scale invasion of Ukraine in 2022, and Moscow currently controls almost 20% of Ukrainian territory.