GAZA CITY (Reuters) - The head of the Hamas government in Gaza urged Egypt on Thursday to open a vital border crossing closed since gunmen killed 16 Egyptian guards in neighboring Sinai on Sunday.

Ismail Haniyeh pledged his support for Egypt's investigations of the attack, which has led to the biggest Egyptian military assault in the Sinai Peninsula in nearly 40 years.

"At the same time I call upon my brother, Egyptian President Muhammad Mursi to open the Rafah crossing, to regain a lifeline for Gaza," Haniyeh said at a dinner ending the daylong Ramadan fast.

"Gaza could never be anything but a source of stability for Egypt," Haniyeh added, saying those responsible for the Sinai slayings were conspiring to tighten an Israeli blockade on Gaza in force since Hamas seized control of the territory in 2007.

Hamas has ruled out suggestions that Palestinian gunmen took part in the Sinai massacre, and has criticized Cairo for imposing "collective punishment" on the impoverished coastal enclave.

The Rafah crossing normally sees some 800 people a day leave for Egypt and beyond, and is the only window on the world for the vast majority of Gazans.

No one has claimed responsibility for Sunday's assault, in which the assailants seized two armored vehicles to storm an Israeli border crossing. One made it through before the attackers were killed by Israeli fire. The incident was the deadliest along the Israeli-Sinai border in decades.

In addition to challenging the durability of a 1979 Israeli-Egyptian peace treaty already strained by Western ally Hosni Mubarak's fall from power last year, the violence also raised questions about ties between Egypt's nascent Muslim Brotherhood-led government and Hamas in Gaza.

Egyptian officials suggested initially that Palestinian gunmen were involved in the Sinai attack.

Haniyeh denied this, arguing that the attack's "prime beneficiary was the (Israeli) occupation."

"Gaza is innocent of this crime. Neither our investigations nor our contacts have proven so far anyone from Gaza was involved," Haniyeh said.