The government of Slovakia said on Friday that it would send 13 Soviet-designed fighter jets to Ukraine, a day after a similar announcement by Poland’s president, marking a possibly significant shift from NATO allies in increasing arms supplies for Kyiv.
Slovakia, which borders both Ukraine and Poland, had said months ago that it was ready to send MIG-29 fighter jets to Ukraine. “Promises must be kept,” Slovakia’s prime minister, Eduard Heger, wrote on Twitter. He did not specify the timing of any delivery.
The news came a day after a surprise announcement by Poland’s president that his country would send four MIG-29 jets to Ukraine within days, a move that appeared intended to open the door to more advanced warplanes from NATO allies. These would be the first warplanes sent to Ukraine by a NATO country since Russia invaded last year.
Ukraine and its Eastern European allies have been pushing for advanced jet fighters, particularly the American F-16. The United States has expressed reluctance, though senior officials have said that the administration had not ruled out deciding later to provide them or, more likely, working out an agreement to allow another country to send them.
At least for now, the United States was standing its ground. The pledge by President Andrzej Duda of Poland “doesn’t change our calculus with respect to F-16s,” John F. Kirby, a White House spokesman, said on Thursday. He said that Mr. Duda had not spoken to President Biden about his plan.
The Biden administration has resisted sending American fighter jets in part because it would take too long to train Ukrainian pilots for military operations underway. It could also prove risky, given Russia’s air superiority.
There are still many questions about how concrete Poland’s plan was. Poland first pledged to send its fleet of 28 MIG fighters a year ago, and the country’s defense officials had been saying recently that they would only send warplanes as part of a coalition with other countries.
On Thursday, Mr. Duda said Poland would transfer four of the MIG warplanes to Ukraine “in the next few days.” Ukrainian pilots are already trained to fly MIG fighters, so the planes could be put into action quickly.
With Russia expected to mount spring offensives, the push to provide more sophisticated weapons has been picking up pace, particularly in Europe’s former Soviet eastern edge, which has been especially vocal about opposing Russia’s aggression.
Other countries have signaled openness to providing fighter jets. Last month, France said it had not ruled out sending fighter jets to Ukraine, though President Emmanuel Macron laid out conditions, including that they not be used “to touch Russian soil.”
Prime Minister Rishi Sunak of Britain said last month that his country would train Ukrainian pilots to fly NATO-standard jets and suggested that he was open to eventually sending planes.
Poland first said it was willing to give its fleet of MIG-29s — thought to number around 28, but not all operational — to Ukraine a year ago, on the condition that the United States would replace them with more modern American warplanes. The plan never came to fruition.
But Poland is still one of the largest donors to the Ukrainian war effort when its support is measured against the size of its economy. On Thursday, Mr. Kirby praised Poland for “really punching above its weight when it comes to supporting Ukraine.”