Two US Senators have asked the Department of Defense to address why US President Donald Trump was using his unsecured Android phone for official communications, media reports said.

According to a report in Engadget on Monday, Senators Tom Carper and Claire McCaskill wrote a letter to Secretary of Defense James Mattis asking for four things.

"First, confirmation on if Trump received a secured, encrypted smartphone for personal use on or before inauguration, and, if not, what kind of phone the President has been using since taking office," the report noted.

The Senators also wanted to know the policies and protections the Defense Information Systems Agency (DISA) has in place for the personal phone's use.

"The senators want to know if the DISA, Secret Service an National Security Agency were consulted and coordinated with regarding the president's use of a personal cellphone," the report added.

It is deeply troubling for US Senators and other security agencies that Trump still uses his unsecured smartphone.

Trump is being urged by his aides and the Secret Service to give up his "unsecure smartphone and stick to the secure one that was issued to him".

"The absolutely minimum Trump could do to protect our nation is to use a secure device to protect him from foreign spies and other threats," New York Times quoted Senator Ron Wyden, a Democrat from Oregon on the Intelligence Committee, as saying.

It was not clear if Trump's device and its functions like texting are encrypted to thwart hacking. The device could be vulnerable to hacking if used on unsecured Wi-Fi and cellular networks.