Gay rights supporters in Washington state are claiming victory as early election results show voters approving a bill that would give same-sex couples equal rights as those traditionally married.

The "everything but marriage" measure gives registered domestic partners additional state-granted rights, with the exception of marriage.

The expanded law would add benefits such as the right to use sick leave to care for a domestic partner and rights related to adoption, child custody and child support.

Opponents have warned that if the domestic partnership measure passed, gay marriage could be the next step in the state.

Just over 51 percent of voters were approving the bill, officially known as Referendum 71, in the initial night of ballot counting.