The Pentagon said Tuesday Libyan forces fighting the Islamic State have been "spectacularly successful" in retaking territory and pushing the Islamic State deep into its stronghold in the northern coastal town of Sirte, which is why the U.S. is helping the effort.

The United States began airstrikes Monday to provide close air support for fighters aligned with Libya's newly formed Government of National Accord, or "GNA" which is supported by the U.S. and other Western nations. Five strikes were conducted Monday, and two more Tuesday, according to the Pentagon.

The Pentagon is describing the mission as short-term but open-ended, depending on the success of the GNA forces in routing Islamic State fighters.

"The objective to to help the GNA take Sirte," said Pentagon Spokesman Navy Capt. Jeff Davis. "We have seen over the past few months as the GNA and their forces, have worked to defeat ISIL, they have been spectacularly successful."

The Pentagon says the number of Islamic State fighters in Northern Libya, which once numbered in the thousands, is now measured in the hundreds.

The GNA forces are now said to control much of Libya coast along the the Gulf of Sidra between Tripoli and Benghazi.

"As they have gotten that far, they are now at the point where they need help getting across the finish line," said Davis. "Where they make advances, they are beaten back, and this is a chance for us to help them get across the finish line."

Davis cited the example of a single Russian-made T-72 tank that was hidden in a wooded area, and was blocking an advance by GNA troops. It was destroyed by a U.S. airstrike, and the Libyan forces quickly moved in.

Davis predicted the Islamic State will be pushed out of Sirte within weeks, not months, but said the Islamic State would still be a threat.

"I don't think that retaking Sirte will mean that ISIS is going to be scrubbed completely out of Libya for evermore," he said.