The US Navy has commissioned shipbuilder Austal to build another two Littoral Combat Ships, adding another $US681.7 million to the company's order book.

Austal said the contract options fund construction of the LCS 14 and LCS 16, the fifth and sixth ships in the 10-ship block buy award made to an Austal-led team in December 2010.

That 10-ship program is potentially worth more than $US3.5 billion.

Austal chief executive Andrew Bellamy said the company's US Navy programs provided guaranteed revenue and workload for years to come.

"Austal's production of these revolutionary ships continues to expand and improve, simultaneously delivering better program outcomes for the Navy and improved financial results for the company's shareholders," he said.

Austal's US president Craig Perciavalle said the award confirmed the Navy's commitment to the LCS program and its confidence in Austal.

Austal has also been contracted by the US Navy to build ten 103-metre JHSVs under a 10-ship, $US1.6 billion contract, one of which has already been delivered.

Construction of another three JHSVs and two LCSs is also underway at Austal's shipyard in Mobile, Alabama.