(CNN) -- Tourists visiting an Egyptian resort have been warned to be alert after a series of shark attacks that have left one woman dead and several people seriously injured.
The German woman died Sunday after being attacked by a shark in waters off Sharm el-Sheikh, prompting Egypt's tourism ministry to close the beach until the animal responsible is found.
Thousands of tourists flock to Sharm el-Sheikh each year, attracted by its warm climate and clear waters, but the UK Foreign Office on Monday amended its travel advice for people visiting the area.
A statement on its website read: "Attacks by oceanic white tip sharks are extremely rare and shark attacks of any kind are very unusual in the Red Sea.
"If you are considering diving or snorkelling in any of the Red Sea resorts be aware that safety standards of diving operators can vary considerably.
"A basic rule is never to dive or snorkel unaccompanied. Where possible make any bookings through your tour representative.
"Unusually cheap operators may not provide adequate safety and insurance standards."
Tour operators Thomson and First Choice also warned holiday-makers about the shark attacks. "We are treating the situation very seriously and are advising all of our customers not to swim in the sea at Sharm el-Sheikh," a statement said. "We have also cancelled all water-based excursions until further notice."
After the attacks authorities ordered people to stay out of the water around Sharm el-Sheikh. On Thursday, the Environment Ministry said two sharks suspected of the maulings were caught.
Jochen Van Lysebettens, general operations manager of the Red Sea Diving College in Sharm el-Sheikh, said employees at the Hyatt Regency resort told him the attack happened about noon (5 a.m. ET) in a protected swim area off the resort. Van Lysebettens has three dive-instruction operations in the area, including one at the Hyatt Regency.
The 70-year-old woman, a regular guest at the resort, was snorkeling near a reef when she was attacked, he said. She called for help, and a lifeguard brought her to shore, but she had lost too much blood and resuscitation efforts failed, Van Lysebettens said. The woman's arm and leg were severed, he said.
A British tourist described witnessing one of the shark attacks. Ellen Barnes, 31, of Horsham, West Sussex, told The Sun newspaper: "The water was churning like I was in a washing machine. I was being thrown around in the blood."
After the Sunday incident, the Chamber of Diving and Watersports called on its members in the region "to stop any snorkeling activities happening from any boats or shore." The chamber is under the umbrella of the Egyptian Tourist Federation.
Van Lysebettens said all tourists were ordered to stay out of the water at least through Monday. The dive community is organizing volunteer expeditions to find the shark, he said, and about 40 people will search Monday.
On Friday, two sharks were caught and killed near the South Sinai National Park on the Sharm el-Sheikh coast, according to the Chamber of Diving and Watersports.
The chamber said that as of Friday, the condition of three injured snorkelers -- two Russian women and a man from the Ukraine -- were unchanged. One of them was critical, the chamber said in a statement, but did not say which one. The three were attacked in a 24-hour period November 30 and December 1, the chamber said.
Van Lysebettens said he is not a marine biologist, but many have speculated an oceanic white-tipped shark was responsible for the German woman's death, as well as the other attacks. Divers are upset because the two sharks killed Friday are thought to be innocent. They were a mako shark and another oceanic white-tipped shark -- which did not match pictures taken by someone accompanying one of the injured snorkelers, he said.
Officials at the national park did not provide details on "why the animals could not be relocated to remote waters as was previously suggested," the chamber's statement said, adding it "does not wish to see any harm to any further sharks."
Exploratory dives were taking place on Friday, Van Lysebettens said.
Officials are not sure what triggered the feeding behavior, described as highly unusual.
"This incident has clearly shocked our community, and the CDWS is continuing its investigation into why this may have happened," Hesham Gabr, head of the Chamber of Diving and Watersports, said in the statement. "It is clear from our initial discussions with shark behavioral experts that this highly unusual spate of attacks by an oceanic white tip shark was triggered by an activity, most probably illegal fishing or feeding in the area."
Several divers in the area saw carcasses of dead sheep in the water last week, Van Lysebettens said. It's not known how they got there, he said, but they could have washed out to sea or fallen off a boat. The shark may have been attracted by the carcass, he said, and could now be attacking slow-moving things on the water's surface.
CNN's Brian Walker and Karen Smith contributed to this report.