WASHINGTON (AFP) - The chaos in Syria will only get worse and destabilise the entire region if the global community fails to act, key figures of the war-torn country's opposition said Tuesday.

Only Washington can deter President Bashar al-Assad's use of chemical weapons and so the US Congress should give the White House the go-ahead to target the strongman, they added.

Mr Ahmad al-Jarba, president of the National Coalition of Syrian Revolution and Opposition Forces, and Mr Salim Idriss, chief of the rebel Free Syrian Army, wrote an opinion piece in the Washington Post.

The pair warned that "international apathy and indecision convinces Assad that he is invincible and emboldens him to unleash barbaric horrors on a defenseless population." And if his actions are left unchecked, the situation is likely to get even worse, they said.

"For all its horror, the situation today is minor compared with what could still happen if Assad is not deterred or held accountable for his crimes," they wrote.

Declaring the two-and-a-half-year conflict an "unprecedented humanitarian catastrophe," they cautioned that despair would breed radicalism.

The longer Mr Assad is allowed to remain "out of control," the more extremists will take advantage of the situation and "the less stable the entire region will become."

"So far, global inaction and (Assad's) allies' protective actions have granted him impunity to terrorise his nation and the region," they wrote.

"Dithering by the world's most powerful states empowers not only the vicious Assad regime but also the extremist agenda of the al-Qaeda-style terrorists seeping into Syria from the east."

"They are fighting not only Assad but, more important, also those who oppose Assad."

Turning to Congress, the two urged US lawmakers to allow President Barack Obama to take action, saying millions of lives and the safety of the region are at stake.

"Please authorise President Obama to act against Assad and to stop him in his deadly tracks," they wrote.

"With the death toll well past 100,000, our struggle to liberate our country from this murderous regime and to protect our people continues, but only the United States can deter Assad from using his chemical weapons again."

According to US intelligence, a chemical attack against rebel-held suburbs of Damascus killed more than 1,400 people, including 400 children, on August 21.

Mr Obama - who faces an uphill battle as he seeks congressional approval for limited military action in Syria - is due to give a prime time address to the American people on the topic later on Tuesday.

The US leader has argued that a military strike is necessary to defend the long-established international taboo against the use of chemical weapons.