SUVA, Feb 23 (Reuters) - Pacific island leaders arrived in Fiji on Thursday for a two-day meeting to formalise the return of Kiribati to the 18-member regional bloc, amid tensions between China and the United States in the strategically located region.

The meeting will also discuss Japan's plan to release treated water from the destroyed Fukushima nuclear plant into the sea, which some Pacific islands fear could contaminate fish stocks but Tokyo has said does not pose a risk, an agenda showed.

Kiribati split from the Pacific Islands Forum on the eve of its last meeting in July, fuelling concern the atoll nation with vast fishing grounds located 3,000 kms (1,860 miles) southwest of Hawaii, had become isolated after forming diplomatic ties with Beijing.

"Set against the backdrop of increasing geopolitical interests in our region, coupled with the real threats posed by climate change... solidarity in our Pacific family is ever so critical," Fiji's prime minister and forum chairman Sitiveni Rabuka said in a statement ahead of the meeting.

In May, China sought but failed to secure a 10-nation Pacific islands trade and security deal, after the forum said unity was the priority in a region where several states have ties with Taiwan and not Beijing.

Australia and New Zealand, both forum members, have criticised China's push for a policing role in the region.