WASHINGTON - President Donald Trump plans to nominate Patrick Shanahan, a former Boeing Co executive, as his defense secretary, the White House said on Thursday, breaking with tradition by choosing someone who made a career at a top defense company as Pentagon chief.

Shanahan had been under investigation by the Pentagon inspector general for allegedly seeking preferential treatment of Boeing while at the Defense Department but he was cleared of wrongdoing in April. He has been acting defense secretary since January, the longest in Pentagon history.

"Based upon his outstanding service to the country and his demonstrated ability to lead, President Trump intends to nominate Patrick M. Shanahan to be the secretary of defense," White House spokeswoman Sarah Sanders said in a statement.

Early tests for Shanahan will include handling growing tensions with Iran, renewed missile tests by North Korea and questions about how the United States should handle the political and economic crisis in Venezuela.

Shanahan said in a statement that he was committed to modernizing US military forces and if confirmed would aggressively implement Trump's national defense strategy.

Shanahan, 56, was thrust into the role in an acting capacity in January, after Jim Mattis abruptly resigned over policy differences with Trump.

The Trump administration has announced its intention to withdraw most US troops from Syria and diplomatic efforts are underway to bring an end to the 17-year-old war Afghanistan.

But the most pressing short-term issue is likely to be Iran.

Earlier this week the Pentagon said it was sending a carrier strike group and bombers to the Middle East after US intelligence signaled possible preparations by Tehran to stage attacks against US forces or interests.

Shanahan is expected to be confirmed by senators, though he could face a tough confirmation process because of his tenure at Boeing.