Barack Obama attempted to dampen talk of an escalating US ground engagement in Iraq on Wednesday as an increasingly nervous Congress prepared to vote on authorisation for his separate plan to arm Syrian rebels.

In contrast to the chairman of the joint chiefs of staff, general Martin Dempsey, who warned a Senate hearing on Wednesday that US troops could fight alongside Iraqi forces in future, the president insisted his prohibition on any American ground combat role would remain.

“I want to be clear; the American forces that have been deployed to Iraq do not and will not have a combat mission,” said Obama in a speech to troops at US Central Command headquarters in Florida.

“As your commander in chief, I will not commit you and the rest of our armed forces to fighting another ground war in Iraq,” he added. “After a decade of massive ground deployments, it is more effective to use our unique capabilities in support of partners on the ground so they can secure their own countries’ futures. That’s the only solution that will succeed over the long term.”

Obama insisted instead US forces would only “support Iraqi forces on the ground as they fight for their own country against these terrorists”.

Though in practice the dividing line between advisers supporting Iraq forces and fighting alongside them may be prove narrow, the language was markedly different from that of Dempsey, who told the Senate armed services committee that he could see himself recommending the use of some US military forces now in Iraq to embed within Iraqi and Kurdish units to take territory away from Isis.

“If we reach the point where I believe our advisers should accompany Iraqi troops on attacks against specific [Isis] targets, I will recommend that to the president,” Dempsey said, preferring the term “close-combat advising”.

“At this point, his stated policy is we will not have US ground forces in direct combat,” Dempsey said, to include spotting for US air strikes. “But he has told me as well to come back to him on a case-by-case basis.”

The apparent divergence in US strategy for combating Isis came as the House of Representatives prepared to vote on authorisation requested by Obama to arm and train Syrian rebel groups.

Though expected to pass with backing from both Republican and Democratic leaders, a number of members in Congress remain concerned at whether the strategy will work, especially since these groups have hitherto primarily been fighting the Assad regime rather than Isis.

There are also grumblings on Capitol Hill about plans to add the vote as an amendment to a much larger budget measure, known as a continuing resolution – effectively holding the funding of the US government hostage if there is serious opposition.

Moderate Democrats in the Senate, such as West Virginia senator Joe Manchin, also voiced their opposition to the authorisation on Sunday on the grounds that arms may end up in the wrong hands.

“I have not come to this decision easily and I know it comes with consequences,” said Manchin, who also called for the vote to be separated from the continuing budget resolution.

But Obama used his speech at CentCom in Tampa to further bolster his justification for taking military action against Isis.

While repeating the acknowledgement that US intelligence had “not yet detected a specific [Isis] plot” against the US homeland, Obama repeatedly spoke of its wider threat to national security.

“If left unchecked they could pose a growing threat to the US,” said Obama.

“Whether in Iraq or Syria, these terrorists will learn the same thing the leaders of al-Qaida already know: we mean what we say, our reach is long, if you threaten America, you will find no safe haven, we will find you eventually,” he added to loud cheers from the troops.

He also claimed the mission was “vital” to US security interests. “Any mission involves risk … sending our service members into harms way is not a decision I ever take lightly. It is the hardest decision I ever make as president;’ nothing else comes close,” said Obama. “I do it only when I know the mission is vital to the security of this country.”

But while Obama was stressing that the US would not act alone against Isis, secretary of state John Kerry was expected to come under pressure at a hearing of the Senate foreign relations committee to give more detail of the alliance that the White House claims to have assembled amid doubts at the extent of international military commitment.