By Maayan Lubell and Mostafa Hashem

JERUSALEM/CAIRO (Reuters) - Militants in Egypt's Sinai peninsula fired rockets into southern Israel on Friday in an incident that caused no casualties but appeared to be linked to fighting between Islamist insurgents and Egyptian security forces.

Islamic State's Egypt affiliate, Sinai Province, said in a statement posted on Twitter by supporters that it had launched three Grad rockets towards "occupied Palestine". Reuters could not immediately verify the authenticity of the statement.

An Israeli military source earlier said the rockets had been fired from Sinai, which borders Israel, the Gaza Strip and the Suez Canal.

Israeli police said they had so far found remnants of two rockets in an open area. No damage or casualties were reported.

It was possible the launch was linked to the fighting in Sinai, the military source said, where militants of the Sinai Province group launched their biggest assault in years against Egyptian security forces on Wednesday.

Dozens of people have been killed in the past few days of clashes and air strikes in the desert region.

The Israeli military on Friday closed a southern highway, part of which runs along the Egyptian border, as a safety precaution.

Egyptian security sources earlier told Reuters they were investigating the reports and said there was no immediate evidence a rocket was launched from Egyptian territory.

Militants, including those from Sinai Province - formerly known as Ansar Bayt al-Maqdis - have fired rockets into Israel from Sinai on several occasions in the past.

Israel has accused Hamas, the Islamist group which controls Gaza, of helping Islamic State in Sinai, an allegation Hamas denies. An Egyptian security source said some Hamas members were involved in the Sinai battles, but that there were no wider organisational ties between the groups

Hamas, an Islamist group that seized power in Gaza in 2007, has faced its own threats from Salafist and Islamic State-linked militants.

On Tuesday Islamic State militants released a video threatening to turn Gaza into another "fiefdom", as in parts of Iraq and Syria.

It also said in the video that it will uproot "the state of the Jews" (Israel) and secular Palestinian movement Fatah, headed by Palestinian President Mahmoud Abbas.