Arbil (Iraq) (AFP) - Militants from the Islamic State jihadist group launched a surprise attack on Kurdish forces in the Gwer area of north Iraq, killing 26, security officials said Saturday.

The militants crossed the Zab river by boat and occupied Gwer -- some 40 kilometres (25 miles) southwest of Kurdish regional capital Arbil -- for about an hour before being pushed back, the officials said.

Accounts differed as to whether the attack began late on Friday or early Saturday.

The 26 people killed were members of the Kurdish asayesh security forces, said Barzan Qassab, the deputy head of the asayesh for Arbil, and the toll was confirmed by another asayesh souce.

Some members of the peshmerga -- the autonomous Kurdish region's other main security force -- were also reported to have been killed.

"It was unexpected," peshmerga Brigadier General Hajar Ismail said of the boat crossing.

IS spearheaded a major militant offensive that has overran large parts of Iraq north and west of Baghdad since June.

The Gwer attack is one of the single deadliest attacks on Kurdish forces since the militant drive began.

After sweeping federal security forces aside and driving south toward Baghdad, IS launched a renewed push against Kurdish forces, driving them back toward their regional capital Arbil.

That helped spark a US-led campaign of air strikes, and Kurdish troops have since managed to regain ground from the militants, including Gwer.