Kamala Harris heads to Poland amid Nato fighter jet rift

US Vice-President Kamala Harris has begun her trip to Poland and Romania, giving her a chance to negotiate directly with Polish leaders on the issue of fighter jet transfers to Ukraine.

The topic has exposed disagreements within the Nato alliance. On Tuesday, Poland's offer to provide the US with MiG-29 fighter jets as an intermediary step toward transferring them to Ukraine was rejected as "not tenable" by American officials.

On Wednesday afternoon, US Defence Department spokesman John Kirby was even more definitive, saying such a transfer presented a "high risk" of a Russian response that leads to a military escalation with Nato.

"We assess that adding aircraft to the Ukrainian inventory is not likely to significantly change the effectiveness of Ukrainian Air Force relative to Russian capabilities," he added.

The statement came after a German government spokesman also dismissed the Polish proposal, describing it as "not currently on the table".

American officials have repeatedly emphasised the US position that the decision on fighters was ultimately one for the Polish government to make.

Polish leaders, however, have said that their nation would only act in co-ordination with allies.

The vice-president's schedule includes meetings with Polish President Andrzej Duda and Prime Minister Mateusz Morawiecki, as well as with Canadian Prime Minister Justin Trudeau, who is in Warsaw for his own visit to a key Nato ally.

Ukrainian President Volodymyr Zelensky has been pressing the Nato military alliance for the Soviet-era jets - which its fighter pilots know how to operate - to bolster its defence against the two-week-old Russian invasion.

"It's been 13 days we've been hearing promises, 13 days we've been told we'll be helped in the air, that there will be planes," he told British politicians this week.

While Nato leaders, including US Secretary of State Antony Blinken, had initially expressed openness to the idea, arranging the details of such a transfer without risking a confrontation between Nato and Russian forces has proven difficult.

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"Poland's proposal shows there are some complexities that the issue presents," Mr Blinken said in a press conference with British Foreign Secretary Elizabeth Truss on Wednesday. "We have to make sure we're doing it in the right way."

Russia, Nato officials worry, could view Ukrainian-piloted fighter jets taking off from a Nato nation - whether it is Poland or a US base in Germany - and heading into combat as a belligerent act on the part of the western alliance.

There are also concerns that, in the face of sustained Russian attacks, Ukraine is running out of operating airfields on its own territory.

Poland hoped to send its Mig-29 jets to Ramstein Air Base in Germany

That reasoning hasn't satisfied some critics in the US, however.

"Biden's decision to block Poland from sending MiG-29s to Ukraine is pathetic," US Senator Tom Cotton tweeted shortly after the announcement. "It's weakness, and the administration's rationale makes no sense. How are jets 'escalatory' when we are already sending Javelin missiles?"

As Mr Cotton notes, other forms of US military assistance to Ukraine - including Javelin anti-tank missiles - are ongoing.

Mr Kirby said the US would provide more air-defence weapons to Ukraine, as well.

US congressional leaders had already reached an agreement to provide an additional $7bn in military hardware to Ukraine and US allies in the region.