WASHINGTON — In a major escalation, President Obama on Wednesday night announced expanded air attacks against ISIS terrorists in Iraq and across the Syrian border — and called for giving military aid to Syrian rebels who would step up the fight against ISIS on the ground.

“I will not hesitate to take action against [ISIS] in Syria, as well as Iraq,” Obama told the nation in a prime-time address from the White House. “If you threaten America, you will find no safe haven.”

Labeling his new push a “counterterrorism campaign” just weeks after saying he didn’t have a strategy yet, Obama said it would be a “steady, relentless effort to take out” the Islamic State (ISIS, or ISIL), a group he called a “cancer” that must be eradicated.

But even as he ordered expanded air assaults against ISIS, Obama acknowledged limits on the effort, stressing coalition-building and a determination not to redeploy ground troops he labored to pull out of Iraq.

“American power can make a decisive difference, but we cannot do for Iraqis what they must do for themselves, nor can we take the place of Arab partners in securing their region,” he said.

Obama said he was dispatching another 475 troops to Iraq — bringing the total stationed there to almost 1,600 — but said US troops would not have a combat mission on the ground.

The CIA already backs the Free Syrian Army in its fight against Syrian President Bashar al-Assad.

But with ISIS terror thugs murdering two American journalists, Obama wants more guns, more training, and a more overt show of support — in part to get other potential allies off the sidelines.

Obama said the United States will lead a “broad coalition” to roll back the terror threat, including Arab nations that “can help mobilize Sunni communities in Iraq and Syria to drive these terrorists from their lands.”

The administration is lifting restrictions on its air campaign in Iraq, working to rub out ISIS wherever possible rather than just where necessary to protect Iraqi minorities and US personnel.

“If there is an ISIL target that we need to hit in Iraq, we will hit it,” a senior administration official said.

“We will not get dragged into another ground war in Iraq,” he vowed, comparing the effort to lower-grade campaigns in Yemen and Somalia.

Obama called on Congress to vote on his plan to have the Pentagon arm the rebels, a strategy that could provide the president a way to claim bipartisan support for — and shield lawmakers from — a possibly unpopular vote.

A more declarative vote on an authorization to broaden the fight to include US airstrikes in Syria — where ISIS has grabbed huge swaths of territory — has been deemed politically dicey by some lawmakers up for re-election.

On Wednesday night, Sen. Ted Cruz (R-Texas) slammed Obama’s “hubris” for laying out a military campaign without a vote of authorization from Congress.

The new measures to train and equip the Free Syrian Army will not be run out of Jordan, as were prior CIA efforts.

The support will go through Saudi Arabia, where the monarchy has been supportive of taking on ISIS. Obama called Saudi King Abdullah Wednesday.

The Saudi kingdom has committed to be a “full partner” in the effort to train and equip the Syrian opposition.

Senate Armed Services Committee Chairman Carl Levin (D-Mich.) said he was “satisfied” Free Syrian forces “can be vetted adequately.”