BEIJING—China is slowing reviews of multibillion-dollar takeover deals being pursued by Qualcomm Inc. QCOM 1.70% and Bain Capital, people familiar with the matter say, as U.S.-China trade tensions escalate.

The delay could end up quashing Qualcomm’s planned $44 billion purchase of Dutch semiconductor company NXP Semiconductors NXPI 1.05% NV—a deal widely seen as critical to Qualcomm’s future—according to a person familiar with the matter.

China is the only country that hasn’t yet signed off on the Qualcomm deal and on Toshiba Corp.’s TOSYY 0.51% planned $19 billion sale of its chip unit to a consortium led by U.S. private-equity firm Bain Capital.

Neither deal is likely to move forward amid the looming trade war, the people said.

“The review process is basically on pause because of the trade tension,” a senior Toshiba official said. “We’ve been afraid of that.”

Stalling these deals is another possible leverage point for China as it seeks to fend off the Trump administration’s plans to impose tariffs on up to $150 billion in Chinese goods in response to what the administration says are unfair trade practices.

China has denied acting improperly and responded with threats of what it calls “teeth-to-teeth” retaliatory measures, including higher levies on $50 billion in U.S. imports.

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China has a say on the Qualcomm-NXP deal as one of several countries where the companies have substantial sales or assets. The deal is considered critical for Qualcomm’s competitiveness, by broadening its reach beyond its stronghold in the handset business, which has reached a plateau.

Qualcomm has said adding NXP would help it reach combined markets, such as connected cars, that it expects to be worth $77 billion by 2020.

These are areas where Qualcomm believes its investments in fifth-generation cellular technology, or 5G, are likely to yield a big payoff. It also expects the merger to add immediately to its adjusted per-share earnings. If the deal falls through, it has said it will buy back shares to boost earnings by an equivalent amount.

Worried that Beijing might scuttle the deal, Qualcomm Chief Executive Steve Mollenkopf raised the issue with China’s vice president, Wang Qishan, in a March 27 meeting, people with knowledge of the event said.

Mr. Wang sought to offer some assurances, the people said. He told Mr. Mollenkopf regulators would review the deal through a “science-based process” and that politics would have nothing to do with it.

People familiar with the matter say the deal is still facing resistance from China’s commerce ministry, which has indicated it is likely to seek more information from San Diego-based Qualcomm. The ministry faces a deadline next week to make a decision, according to one of the people. To keep the review alive, Qualcomm and NXP could withdraw the current application and refile for an extension.

Qualcomm faces its own deadline to complete the NXP deal. The companies’ merger agreement provides for two automatic extensions of the deal’s deadline, and the second was triggered in January, extending it to April 25. The companies could still negotiate more extensions, however.

Qualcomm declined to comment. NXP referred any questions to Qualcomm, its acquirer. A Bain representative declined to comment on the Toshiba deal. China’s Commerce Ministry didn’t respond to a request for comment.

A Qualcomm deal is an important part of the company’s plan for revenue growth, after the Trump administration last month quashed a $120 billion proposed acquisition of Qualcomm by Broadcom Ltd. —an outcome Qualcomm wanted.

Qualcomm also faces a long-shot attempt by its former chairman and CEO, Paul Jacobs, to marshal support to buy Qualcomm, which he has said would be better off under private ownership.

As for Toshiba, the longer it holds onto its chip business, the greater its risk of losing its technological edge: To remain competitive in chips, it would have to make frequent multibillion-dollar investments, which would be a challenge for Toshiba with its unsteady finances.

As of April 1, Toshiba gained the right to cancel the deal with the Bain-led consortium under the original sale contract. Toshiba executives have said they want the deal to go through, but some shareholders have said it should be scrapped. A lengthy delay in China would increase that likelihood.

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Toshiba’s financial situation has improved since getting hammered last year by the bankruptcy of U.S. subsidiary Westinghouse Electric Co. A delay in the sale might even give Toshiba a chance to negotiate a higher price reflecting the latest market values. But further delay also means it will take longer for Toshiba to again compete against other rivals, including Samsung Electronics Co.

Washington and Beijing have been embroiled in a trade spat that has become a source of financial-market turmoil in recent weeks and raised concerns that a full-bore trade war could drag down the global economy.

Having already tightened scrutiny of inbound deals from China, U.S. officials are also crafting sharp prohibitions on Chinese investment in advanced U.S. technology, taking aim at China’s practice of using state subsidies to foster Chinese dominance of future frontiers of manufacturing.

Beijing, for its part, has blamed the U.S. for wrecking the global trade order and is campaigning to line up support from other countries, especially in Europe, whose companies could be given better access to China’s markets should China react to stepped-up pressure from Washington by retaliating against the U.S.

So far, China has threatened to impose stiff levies on U.S.-made soybeans, sorghum and other products, mainly from Farm Belt states that helped Mr. Trump win the 2016 election. And it is looking for retaliatory options beyond tariffs, government advisers and China analysts say, such as stepped-up regulation of U.S. companies in China.

Putting off approvals of cross-border deals that could benefit U.S. firms is bound to further rattle policy makers in Washington as the two sides enter the next phase of high-stakes dance.

“Merger reviews and decisions should be based on consistent, scientific, market-based calculations and never the politics of U.S.-China relations,” said Jacob Parker, vice president of China operations at the U.S.-China Business Council.

—Kosaku Narioka in Tokyo contributed to this article

Write to Lingling Wei at lingling.wei@wsj.com, Yoko Kubota at yoko.kubota@wsj.com and Takashi Mochizuki at takashi.mochizuki@wsj.com