* Detained men previously jailed for resort bombings - security source
* Jihadi gunmen killed 16 Egyptian border guards on Sunday
* Attack prompts Egyptian army crackdown on Sinai militants
* Egypt reopens Rafah border for those returning to Gaza
By Shaimaa Fayed
CAIRO, Aug 10 (Reuters) - Egyptian military forces have captured six "terrorists" in the Sinai region after an attack on a police station earlier this week that killed 16 border guards near the frontier with Israel, state media quoted a military source as saying on Friday.
Egypt sent hundreds of troops and armoured combat vehicles into North Sinai on Thursday to tackle militants operating near the border and commanders said as many as 20 people they deemed terrorists had been killed in the offensive.
The campaign, which Cairo billed as its biggest military operation in the desert region since the 1973 war with Israel, is offensive is crucial to the maintenance of stable relations prevailing since the two countries' 1979 peace treaty.
But some Sinai residents were sceptical about the reported crackdown, saying they had seen no sign of anyone being killed in what they described as a "haphazard" operation.
The military source told state television that six militants had been seized in the border settlement of Sheikh Zuwaid, where on Wednesday Egyptian warplanes fired rockets at suspected militant hideouts.
A security source in North Sinai told Reuters that seven men had been detained but that figure could not be immediately corroborated.
He said the detained men were previously arrested after bombings in resorts along Sinai's southern Red Sea coast between 2004-2006 that killed or wounded hundreds of foreign tourists, and were jailed for months before being freed without charges.
Israel fears Islamist militants based in the increasingly lawless Sinai could link up with Palestinian jihadis in the neighbouring Gaza Strip to launch attacks on the Jewish state - potentially jeopardising the peace accord.
Disorder has been spreading in North Sinai, a region with many guns and bristling with resentment over neglect by Cairo, since the overthrow of President Hosni Mubarak in February in a popular uprising last year. Mubarak's government had worked closely with Israel to secure the frontier region.
Newly elected Egyptian President Mohamed Mursi, who took office in June, has promised to restore stability.
He brushed aside accusations that his background in the Muslim Brotherhood, and ideological affinity with the Islamist Hamas rulers in Gaza, might lead him to take a softer line on militants bent on the destruction of Israel.
Mursi sacked the intelligence chief on Wednesday and announced other changes in the security appointments.
Ismail Haniyeh, head of the Hamas government in Gaza, pledged support on Thursday for Egypt's investigations into the attack on Sunday, but urged it to reopen a vital border crossing closed since then.
Hamas has ruled out suggestions that Palestinian gunmen took part in the Sinai massacre and have criticised Cairo for imposing "collective punishment" on the impoverished Mediterranean coastal enclave by sealing the frontier point.
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.
Egypt opened the Rafah border on Friday to allow the return of pilgrims from Saudi Arabia and those stranded on their way back to Gaza, the state news agency said, citing a high-ranking official, without outlining how long it would stay open.
Around 200 people had been allowed into Gaza through the border gate by 1000 GMT, a Sinai security source told Reuters.
Egypt has moved to seal myriad smuggling tunnels connecting Sinai and the Gaza Strip since Sunday's attack. The Egyptian state newspaper al-Ahram said on Friday some 150 tunnels had been destroyed. The tunnels are believed to number some 1,000.
"We want this crossing to remain open for goods and for people. When this happens there will be no need for tunnels," senior Hamas official Ahmer Bahar told worshippers at a Gaza mosque on Friday.
"Tunnels were an exceptional measure ... When we have a free trade corridor, this issue will come to an end," he said.
Tunnellers on the Egyptian side said Egyptian forces were not closing tunnels that were previously known to them and used to bring food and construction materials to Gaza.
Israel has welcomed Egypt's offensive while continuing to express worries about the deteriorating situation in Sinai, home to anti-Israel militants, disgruntled Bedouin tribes, gun-runners, drug smugglers and al Qaeda sympathisers.
Israel says Palestinian jihadi groups have been crossing from Gaza into Egypt and exploiting the security vacuum there by teaming up with local militants with the aim of attacking Israel's long border running south to the Red Sea.
Tourism in Sinai, especially along its Red Sea coast, has risen over the past decade but security was tightened after the deadly bombings in 2004-2006 by Islamist militants from the north who infiltrated across its wild mountainous interior.