(CNN) -- Security forces in Yemen killed four people when they opened fire on anti-government protesters on their way to a funeral Friday in southwestern Yemen, medical workers and witnesses said.

At least 12 other protesters were shot, with three of them in critical condition, medical workers said.

The security forces were trying to disperse the crowd gathered in Ibb province to mourn a protester who was killed on Wednesday, the medical workers and witnesses said. The province is about 120 miles from the capital, Sanaa.

Yemen is among the Arab countries swept up in protests against longtime rulers since the January revolt that ousted Tunisian strongman Zine El Abedine Ben Ali. The impoverished and unstable nation has been wracked by anti-government protests and clashes between demonstrators and security forces for many weeks.

"We were walking to the graveyard to bury our dead, and the government killed three more," said a protester who attended Friday's funeral. "This is the strategy of a dictatorship ruler."

The protester spoke before the fourth death was confirmed Friday. Medical workers have been trailing protesters in ambulances, anticipating violence.

CNN is not able to independently verify the witness accounts.

On Yemen TV, President Ali Abdullah Saleh said, "The opposition want to destroy the Yemeni economy, but we will not allow them to do so.

"All Yemenis must unite to stand against the crises," he said.

Yemen is "on the verge of an economic catastrophe due to ongoing political unrest and frequent bomb attacks against oil pipelines," Saba quoted Oil and Minerals Minister Amir al-Aydarous as saying Thursday.

"Acts of sabotage against the pipeline in Wadi Ubaida in Marib have hampered the flow of oil since mid-March and undermined the trust of investors in the country," Aydarous was quoted as saying. "Several oil companies have quit the country and the refineries in the southwestern governorate of Aden came to a standstill a week ago," he told the Yemeni parliament, according to Saba.

Minister of Trade and Industry Hisham Sharaf has estimated that the protests have cost the nation's economy $5 billion, or about 17 percent of Yemen's 2009 gross domestic product, Saba said. GDP is the market value of all final goods and services produced in a given period.

On Friday, Saleh's regime reacted positively to news that Qatar had withdrawn from efforts to end the upheaval in Yemen. The Persian Gulf emirate walked away over Saleh's reluctance to leave the country to sign a deal that Qatar had helped negotiate between Saleh and opposition leaders through the six-nation Gulf Cooperation Council.

As protests against his 32-year rule mount in Yemen, Saleh has said he won't leave the country to sign the deal because he fears a coup, a senior official of his ruling party said last week.

"Yemen is ready to deal positively with the Gulf initiative without the participation of Qatar, according to Saba, Yemen's state-run news agency.

"It has become clear that Qatar has its own agenda," Saba quoted an official source at the General People's Congress and the National Democratic Coalition Parties as saying.

The Gulf Cooperation Council is made up of Bahrain, Kuwait, Oman, Qatar, Saudi Arabia and the United Arab Emirates.

The agreement says that Saleh will transfer power and leave office within 30 days. It provides immunity for him and those who served in his regime and calls for a unity government to be formed within seven days.

Saleh wanted an aide to go Saudi Arabia and sign the deal on his behalf. But Saleh's opposition threatened to back out of the deal if he did not personally sign the agreement.