President Obama told Congress that it's impossible to say how long U.S. troops will have to be deployed to fight the Islamic State of Iraq and Syria, in a mandatory update he sent to Congress Thursday evening.

"It is not possible to know at this time the precise scope or the duration of the deployments of U.S. Armed Forces necessary to counter terrorist threats to the United States," Obama wrote in a letter accompanying a classified report to House Speaker John Boehner, R-Ohio, and Senate President Pro Tem Orrin Hatch, R-Utah.

Obama also said more troops could be deployed if needed. "If necessary, in response to terrorist threats, I will direct additional measures to protect U.S. citizens and interests," he wrote.

That message was sent just a day after Obama announced that 450 more military advisers would be heading to the Middle East.

"In furtherance of U.S. counterterrorism efforts, the United States continues to work with partners around the globe, with a particular focus on the U.S. Central Command's and U.S. Africa Command's areas of responsibility," Obama added. "In this context, the United States has deployed U.S. combat-equipped forces to enhance the counterterrorism capabilities and support the counterterrorism operations of our friends and allies."

According to the letter, the U.S. has at least 16,750 "combat-ready" troops deployed in 12 countries or regions: Afghanistan, Cuba, Djibouti, Egypt, Iraq, Jordan, Kosovo, Niger, Somalia, Syria, Yemen and central Africa. Not all of the deployments had figures that were made public.

The three largest contingents are in Afghanistan, Iraq and Jordan, with 9,100, 3,550 and 2,200, respectively. It said "up to 300" military personnel searching are central Africa for Joseph Kony and other top leaders of the Lord's Resistance Army. The International Criminal Court indicted Kony, a Ugandan warlord known for enlisting thousands of child soldiers, in 2005 on war crimes and for crimes against humanity.

Obama's letter was sent pursuant to the War Powers Resolution, which requires updates about ongoing military action every six months. It also forbids a president from committing U.S. troops for more than 60 days without congressional approval or a war declaration, although neither have happened yet.

For several months now, the White House and Congress have been unable to agree on a new Authorization for Use of Military Force to outline what Obama is allowed to do to fight the Islamic State. The White House has called on Congress to act on an authorization it sent four months ago, but Republicans say the White House needs to try again with a more aggressive plan.

That has left the discussion stuck, and has prompted some lawmakers to concede that efforts to write a new AUMF are futile. The administration has said it doesn't necessarily need new authorization, as it can use existing authorities, although it has said a new AUMF would show that Congress and the White House are united.

Obama has relied on the 2001 AUMF to counter ISIS, even though it was first granted to fight al Qaeda in Afghanistan.