The United States on Tuesday rejected a Polish offer to send MiG-29 fighter jets to Ukraine via a US air base, saying the proposal raised "serious concerns" for the entire NATO alliance.
Warsaw caught US officials off guard with the offer to deliver the Soviet-era planes to the US base in Ramstein, Germany.
Under the proposed scheme, those jets could then be deployed to Ukraine, while the Polish air force would receive F-16 fighters as replacements.
Pentagon spokesman John Kirby said the prospect of the jets, placed at the disposal of the United States, flying from a US-NATO base "into airspace that is contested with Russia over Ukraine raises serious concerns for the entire NATO alliance."
"We will continue to consult with Poland and our other NATO allies about this issue and the difficult logistical challenges it presents, but we do not believe Poland's proposal is a tenable one," Kirby said in a statement.
"It is simply not clear to us that there is a substantive rationale for it," he added.
Ukraine's President Volodymyr Zelenskyy has urged British MPs to designate Russia as a "terrorist state" after President Vladimir Putin ordered a special military operation against his nation and called for tougher sanctions on Moscow to "make sure our skies are safe".
The 44-year-old Ukrainian leader, who made a "historic" address to the House of Commons via videolink on Tuesday, received a standing ovation by members of Parliament.
"We are looking for your help, for the help of Western counties. We are thankful for this help and I am grateful to you, Boris," said Zelenskyy, addressing British Prime Minister Boris Johnson.
"Please increase the pressure of sanctions against this country (Russia) and please recognise this country as a terrorist state. Please make sure that our Ukrainian skies are safe. Please make sure that you do what needs to be done and what is stipulated by the greatness of your country. Glory to Ukraine and glory to the United Kingdom," he said.
