Lebanon's Hezbollah movement on Friday said a US pledge to keep its troops in Syria to defeat the Islamic State group was just a "flimsy excuse" to occupy the country.

US Secretary of State Rex Tillerson said Wednesday that US forces would remain in Syria to both fight IS and counter the influence of President Bashar al-Assad.

Assad is a key ally of the Iranian-backed Hezbollah, which has deployed its forces to keep the Damascus regime in power.

During a televised address to commemorate Hezbollah fighters killed in Syria, the group's chief Hassan Nasrallah fired back at the US.

"The Americans are the last people to have anything to do with rolling back Daesh," he said, using the Arabic acronym for IS.

The US, according to the Hezbollah leader, was "creating flimsy excuses to keep their forces and bases in the region. This is the real aim."

The United States has deployed around 2,000 ground troops to Syria and its warplanes patrol the skies over the east of the country, hunting IS remnants.

In a speech on Wednesday at Stanford University, Tillerson said the US "will maintain a military presence in Syria, focused on ensuring that ISIS cannot re-emerge".

But he also said the open-ended deployment is intended to help create conditions for Syrians to be able to remove Assad from office and reject Iranian influence.

The US has long considered Hezbollah a "terrorist" organisation and has targeted it with sanctions.

Last week, the US Justice Department announced it was also creating a special task force to investigate Hezbollah's alleged involvement in the international drug trade.

Current and former US officials have described a massive money-laundering operation involving drugs and used cars that they say has helped Hezbollah fund its operations.

But Nasrallah vehemently denied the claims on Friday.

"These are unjust accusations, that are not based on facts or truth," he said.

"Hezbollah has a very clear religious, jurisprudent, moral position on this. For us, dealing drugs is forbidden (in Islam) and not allowed," Nasrallah said.

He accused American security services and the CIA of "destroying societies" by spreading drug use abroad.

"Have a committee investigate your own involvement," Nasrallah said.