The US Department of Justice has asked the Federal Communications Commission to hold back on Softbank’s proposed $20 billion bid for Sprint Nextel while the DoJ conducts its own investigation.

The DoJ, FBI and Department of Homeland Security have asked the FCC for more time to conduct their own review of the Softbank-Sprint deal for any national security, law enforcement and public safety issues, according to a filing on the FCC’s website.

Sprint’s related bid to acquire Clearwire is also facing opposition. Rival Verizon Wireless earlier this week made a filing with the FCC in which it argued that the regulator should apply its spectrum cap to Clearwire’s 2.5 GHz spectrum in the same way as spectrum in other bands.

If the regulator accepted Verizon’s arguments then it would limit  how much spectrum Sprint could acquire via the Clearwire deal. Sprint has argued because the spectrum is above 2 GHz then the cap does not apply.

The Sprint-Clearwire deal is linked to Softbank’s proposed acquisition of a 70 percent stake in Sprint. Verizon Wireless did not comment on the latter deal.

Further opposition to Sprint comes from Crest Financial, which owns an eight percent stake in Clearwire, making it the second-largest shareholder after Sprint itself. Crest has formally asked the FCC to block both deals involving Sprint.  Separately, it is suing Clearwire in an attempt to block the deal.

An investigation by the Department of Justice is standard in a deal such as Softbank-Sprint which involves foreign ownership of a US operator. However concerns have been raised because Softbank has worked with Chinese vendors Huawei and ZTE in its home market of Japan.

The US security establishment has increasingly voiced its concerns about the presence of Huawei and ZTE in the US telecoms market because of their relationship to the Chinese state.

The FCC is currently taking comments on the Softbank-Sprint deal until 25 Feb and is approximately one third through the informal six-month schedule for looking at the acquisition.