Australian Prime Minister Tony Abbott renewed his nation's commitment to keep up the search for the missing Malaysian jetliner but warned it had become "an extraordinarily difficult'' task.
"If this mystery is solvable, we will solve it,'' Abbott said of the search for Malaysian Airlines Flight 370. "But I don't want to underestimate just how difficult it is.''
As Abbott addressed a televised news conference, the Australian Maritime Safety Authority said an international force of 10 aircraft and 10 ships was heading back into the search area Monday morning off Perth in the Indian Ocean.
Abbott declined to state how long his nation was prepared to continue the air and sea search, which has thus far failed to recover any debris or wreckage confirmed to be linked to the Boeing 777 aircraft. He said no time limit had been set.
"We can keep searching for quite some time to come, and we will keep searching for quite some time to come,'' he said.
Abbott also said that "absolutely overwhelming" evidence indicates the plane was lost in the south Indian Ocean.
"This is an extraordinarily difficult exercise,'' he said. "The best brains in the world are applying themselves to this task.''
On Sunday, an Australian pilot searching for the plane, which was lost with 239 people aboard, said his crew spotted four orange items that could serve as a "promising lead" in the investigation.
Royal Australian Air Force Lt. Russell Adams told a news conference his crew photographed the items, each more than 6 feet long, and logged the location. Investigators will determine whether the find was sufficiently significant to send a boat into the area.
The search had taken a step back Saturday, when debris pulled from the search area turned out to be fishing gear and other items apparently unrelated to Flight 370, the Australian Maritime Safety Authority reported.
Still, Abbott on Sunday described the "intensifying search effort" as positive. The safety authority said nine planes and eight ships searched almost 100,000 square miles Sunday in relatively clear weather.
The search could be on the verge of taking a scientific turn as the ADV (Australian Defense Vessel) Ocean Shield is scheduled to depart Monday from Perth with a black box detector and an underwater vehicle. However, it will take the ship three to four days to reach the search zone.
The plane, with 239 people aboard, disappeared March 8 shortly after taking off from Kuala Lumpur, Malaysia, en route to Beijing. The failed effort to find the plane has caused an uproar with the families of the missing, most of whom are Chinese.
On Sunday, more than two dozen of those Chinese family members arrived in Kuala Lumpur and immediately held a protest at a hotel, holding up banners that read, "We want evidence, truth, dignity" in Chinese, and "Hand us the murderer. Tell us the truth. Give us our relatives back" in English.
The protesters Sunday chanted slogans in Chinese: "We want evidence! We want the truth! We want our relatives!"
Jiang Hui, the relatives' designated representative, said they wanted a government apology for what they see as missteps in the initial handling of the disaster as well as Prime Minister Najib Razak's statement that indicated the plane had crashed with no survivors. Jiang said the relatives felt the conclusion was announced without sufficient evidence.
"We also request that Malaysia Airlines and the Malaysian government apologize for making the conclusion on March 24, without direct evidence or a sense of responsibility, that the plane was destroyed and people died," Jiang said.
Ong Ka Ting, the Malaysian prime minister's special envoy to China, went to greet the relatives at a hotel.
"I'm sure in Beijing they've already had a lot of discussions and we understand their feelings, and we know that definitely by coming over here there will be a lot more discussions and meetings," Ong said. "So we try our best to assist them."
After Malaysian Prime Minister Najib Razak said this week that the plane went down in a remote area of the southern Indian Ocean, far from any possible landing sites, Malaysia Airlines Chairman Tan Sri Md Nor Md Yusof put out a statement saying there was no hope for survivors.
"Based on this evidence, the prime minister's message was that we must accept the painful reality that the aircraft is now lost and that none of the passengers or crew on board survived," he said.
However, Malaysian Defense Minister Hishammuddin Hussein pledged Saturday to continue the search for "possible survivors."
"Even hoping against hope, no matter how remote, of course we are praying and we are continuing our search for possible survivors," said Hussein, who was meeting with relatives of those aboard the missing flight.
Australian officials shifted their search for the missing Malaysian jetliner Friday, citing "a new credible lead'' about the path of the aircraft and where debris may be located. The new search area is about 1,150 miles west of Perth, Australia, and about 685 miles to the northeast of the previous search area.
There is still no solid evidence regarding what led to the plane's disappearance. Earlier this week, Hussein said there was an ongoing investigation.
"We can confirm that the police have interviewed more than a hundred people, including families of both the pilot and co-pilot," he said.
Contributing: Katharine Lackey in McLean, Va.; Laura Petrecca in New York City; the Associated Press