US Defense Secretary Ashton Carter departed Tuesday for an Asian security summit in Singapore, where Beijing's military expansion across the South China Sea likely will once again dominate discussions.

Regional neighbors are fretting over what they see as China's expansionism as it rushes to exert sovereignty over the waterway, a major global shipping route believed to be home to large oil and gas reserves.

China is using dredgers and other tools to convert low-lying ocean features and sandy blips into military bases.

A Pentagon report this month said China has added more than 3,200 acres (1,300 hectares) of land to the seven features it occupies in the Spratly Islands archipelago.

The so-called Shangri-La Dialogue, an annual meeting in Singapore, will see defense ministers, military chiefs and defense experts from the Asia-Pacific region and beyond discuss regional security issues.

Aside from the South China Sea, delegates are expected to focus on the growing threat of Islamic terrorism in the region and North Korea's nuclear program.

Since becoming President Barack Obama's fourth Pentagon chief in February 2015, Carter has taken a forceful tone on Beijing's South China Sea construction.

He criticized the drive at last year's Shangri-La meeting and on Friday, Carter said China risks creating a "Great Wall of self-isolation."

"Countries across the region -- allies, partners and the unaligned -- are voicing concerns publicly and privately at the highest levels," Carter said.

The United States has conducted several "freedom of navigation" operations where it pointedly ignores China's claims of sovereign exclusion zones around the islands by closely flying or sailing past.

Carter's trip will see him first visit an Army base in Arizona.

He had also considered meeting his Japanese counterpart Gen Nakatani during a visit to Japan, but the two decided to meet in Singapore instead, a US defense official told AFP on condition of anonymity.

The official said the meeting would have been too politically sensitive, given the furor surrounding the arrest of a former US Marine on Okinawa in connection to the death of a 20-year-old woman who had been missing since late April.

A series of crimes including rapes, assaults and hit-and-run accidents by US military personnel, dependents and civilians have long sparked local protests on the crowded island.

Carter and Obama were quick to condemn the crime and offer "regret."

Carter "made the decision to not visit Japan on this trip given that he and Minister Nakatani are already planning to meet in Singapore as well as other scheduling concerns," Pentagon spokesman Commander Gary Ross said.

While in Singapore, Carter will be joined by senior US military leaders, including Chief of Naval Operations Admiral John Richardson and the commander of US Pacific Command, Admiral Harry Harris.