A European plan to give Ukraine another Patriot air defense system to protect its battered cities from Russian airstrikes is coming together, piece by piece.

The radar and three missile launchers are being supplied by the Netherlands. Some interceptor missiles are coming from a four-country coalition led by Germany. A mobile fire control center has been promised, although officials won’t say yet from where or by whom. Additional missiles and launchers, as well as training for Ukrainians to use the sophisticated system, will be provided by as many as eight countries.

“We have all the pieces of the puzzle,” the former Dutch defense minister, Kajsa Ollongren, said in an interview before she left office last week as part of a long-expected transition in the Netherlands’ caretaker government. “We just have to put them together.”

It is the Patriot puzzle, as the assembled air defense system is being called by NATO officials.

President Volodymyr Zelenskyy of Ukraine has said he desperately needs at least seven Patriot batteries to fend off attacks across the country. U.S. President Joe Biden has promised that five Western air defense systems will soon be delivered to Ukraine.

Romania has pledged to give one of its systems, following similar commitments from Germany and Italy. One more is expected from the United States.

The fifth may be delivered via the piecemeal approach.

For months, allies have been scouring their arsenals and settling on a creative, if not surefire, way to provide another Patriot system: Build a complete one out of spare parts donated from around Europe.

“All of us have limited possibilities,” said Ollongren, who devised the plan after Zelenskyy first asked for seven Patriot systems in April. “But if we join forces, I think we can make it happen.”

A spokesperson said the Dutch government will continue the effort under the new defense minister, Ruben Brekelmans, who took office Tuesday.

All of NATO’s member states are hard-pressed to give up any more of the estimated 90 surface-to-air Patriot batteries that weapons trackers at the International Institute for Strategic Studies in London say are scattered across the military alliance — two-thirds of them owned by the United States. For smaller European countries that can ill-afford the $1 billion-plus systems, donating even one from their stocks poses risks to their own national defense.

Yet, presenting additional air defense to Ukraine is likely to be one of the few tangible commitments that the allies will announce this week at a high-level meeting of NATO leaders in Washington — a pledge that may only partially placate Zelenskyy.

“They know that urgently we need seven Patriot systems — yes, to save our cities,” Zelenskyy said at a news conference with Biden at the Group of 7 meeting last month in Italy. “And we discussed the possibility of having five of them, it’s true.”

“It doesn’t mean that tomorrow we will have these five systems, but we see, in the closest future, good result for Ukraine,” Zelenskyy said in English.

It was Zelenskyy’s request for seven systems that had started Ollongren exploring how to provide another Patriot system, even though the Netherlands’ highest- ranking military officer had advised that the Dutch did not have one to spare.

They did, however, have some spare parts available. As it turned out, so did some other NATO nations. And countries that didn’t have any pieces to give but wanted to contribute something nonetheless said they would help to finance or train as many as 90 or so Ukrainian soldiers needed to operate a single battery.

“One of the things that we’re exploring right now is the interoperability of the systems,” the Swedish defense minister, Pal Jonson, said at a NATO meeting in Brussels last month. He said Sweden is considering “a whole range of things that we could contribute to this puzzle,” but he would not explicitly say what.

Sweden and the Netherlands are among only seven NATO countries that field Patriot systems; the others are Germany, Greece, Poland, Romania and Spain, according to data provided by the IISS.

There is a Spanish battery positioned at a Turkish air base under NATO authority, but three others that alliance members had stationed in Slovakia have all been withdrawn by their owners: Germany, the Netherlands and the U.S., according to the IISS data. “We have no system in Slovakia, and we are on the border with Ukraine,” the Slovak defense minister, Robert Kalinak, said at last month’s NATO meeting. Slovakia is negotiating to buy some systems, he said, but “we need some kind of solidarity for the next two years as we gave away all we had.”

Germany’s defense minister, Boris Pistorius, said that with its latest donation, his government had already given what it can, “but others maybe can do more.” Three of the four Patriots already in Ukraine were provided by Germany, which is leading a related effort with the Netherlands, Norway and Denmark to deliver 100 Patriot interceptor missiles in the coming months.

The United States owns the vast majority of the Patriot arsenal in NATO — 62 deployable batteries, according to the IISS data — but is fielding some in the Middle East, mainly to protect U.S. bases and interests from Iranian airstrikes, and in Japan and South Korea. The Biden administration is also redirecting orders for Patriot missiles placed by other countries to focus on getting them to Ukraine.

Henry Boyd, an air defense expert at IISS who tracks Patriot systems worldwide, said the Dutch plan to piece one together could be a limited solution for Ukraine, given that the United States “is overstretched” and the Europeans are “basically at minimum levels already.” But he questioned whether Patriot parts coming from different countries — with a range of aging models and software upgrades — could be made to work together.

“There’s quite a question mark in terms of interoperability,” Boyd said, although Ollongren said the Netherlands has set up a team of technical experts to go to each donor country and help ensure that all of the pieces fit.

Ollongren would not say when the assembled Patriot battery might be delivered to Ukraine and raised the possibility that it might instead be sent in parts to replace broken pieces of systems that are already there.

“There are always challenges,” she said. But after proposing the Patriot puzzle, “everybody was really, first of all, aware that we have to do this for Ukraine, and that everybody can do something.”