The US Department of Justice is urging regulators to delay endorsing Softbank's $20 billion takeover of Sprint Nextel until the national security ramifications are evaluated.
The Japanese mobile carrier in October announced a deal to acquire 70 percent of Sprint Nextel, the number three US carrier behind AT&T and Verizon Wireless, by the middle of this year.
If completed, it would be the biggest overseas acquisition by a Japanese firm.
In a letter sent Monday to the Federal Communications Commission, the DOJ asked regulators there to delay taking action on the deal until "national security, law enforcement, and public safety issues" have been assessed.
The request had the backing of the US Department of Homeland Security along with the Federal Bureau of Investigation, according to the letter signed by Jennifer Rockoff of the DOJ's national security division.
A Sprint spokesman referred to the request as "routine" and said that the company continued to anticipate the completion of the transaction by the middle of this year.
A spokesman for Softbank in Tokyo declined to comment.
"This is not a first," said independent industry analyst Jeff Kagan. "This always happens when foreign companies want into the US marketplace."