The Philippine military says 13 marines have been killed in fierce fighting with Muslim militants who have laid siege to southern Marawi City.
Last Friday, when the June 12 deadline was announced, 13 soldiers were killed by militant snipers, bringing to 58 the number of government troops killed since the fighting began on May 23.
Herrera said the military was "validating" reports that the two Maute brothers who founded the group had been killed.
A USA official, speaking on condition of anonymity, said support included aerial surveillance and targeting, electronic eavesdropping, communications assistance, and training.
Washington deployed special forces soldiers to Mindanao in 2002 to train and advise Philippine units fighting the Abu Sayyaf group in an operation that once involved 1,200 Americans.
Filipino forces repeatedly bombed Islamic militant positions Monday as the national flag of the Philippines was raised to mark Independence Day, a celebration of the end of Spanish colonial rule.
Philippines laws provide that the President of the Republic will serve as the Commander-in-Chief of the Armed Forces of the country and as commander-in-chief.
The slain marines were on an operation to rescue about 100 hostages held by the militants, who set off improvised explosive devices and fired rocket-propelled grenades at the advancing troops, Herrera said.
The announcement of U.S. help in the embattled southern region of Mindanao comes after Duterte has sought to reduce the Philippines' reliance on the United States and build much closer ties with China and Russian Federation.
Nearly the entire population of about 200,000 fled from Marawi after the militants stormed it, but the military believes that beyond the checkpoints now fencing off its main roads there are still some 500-600 civilians trapped or being held hostage.
Mr Abella said: "The fight against terrorism, however, is not only the concern of the Philippines or the United States but it is a concern of many nations around the world".
"People are not in the mood for celebrating", said the bishop, adding that he supported the demonstrations against the declaration of martial law "in my own little way".
Presidential spokesperson Ernesto Abella said in a statement U.S. forces were participating directly in combat operations, which is prohibited by Philippines law.
The government had earlier said the unrest had left 20 civilians, 134 militants and 39 government troops dead.