Shaam News Network, Associated Press
In this image taken from video obtained from the Shaam News Network, posted on April 16, 2014, an anti-Bashar Assad activist group, which has been authenticated based on its contents and other AP reporting, children are seen receiving oxygen in Kfar Zeita, a rebel-held village in Hama province some 200 kilometers (125 miles) north of Damascus. Syrian opposition activists and other witnesses tell The Associated Press that Syrian government forces have attacked rebel-held areas with poisonous chlorine gas in recent months. The head of an international mission to Syria charged with destroying the country's chemical weapons called on President Bashar Assad's government Sunday to ensure it meets a deadline to destroy all its toxic chemicals amid a raging civil war.

DAMASCUS, Syria — The head of an international mission to Syria charged with destroying the country's chemical weapons called on President Bashar Assad's government Sunday to ensure it meets a deadline to destroy all its toxic chemicals amid a raging civil war.

Also Sunday, four more candidates announced their candidacy for Syria's upcoming June presidential election, state television announced, a poll Assad is expected to win.

Meanwhile, clashes raging through the northern city of Aleppo killed and wounded at least 70 people, activists reported.

Sigrid Kaag of the Organization for the Prohibition of Chemical Weapons told reporters in Damascus that 92.5 percent of Syria's chemical materials had been removed from the country and destroyed. She called it "significant progress," although she called on Syria's government to ensure remaining materials would be eradicated by the end of April.

"I strongly encourage (the Syrian government) to go for that last push that we can really talk of hundred percent removal and destruction," Kaag said.

Syria missed an April 13 deadline to destroy all its chemical weapons in accessible locations. International experts say that could impact on reaching a June 30 deadline to remove all Syria's chemical weapons.

"An important (achievement) has been made in permanently closing down production facilities," Kaag said, adding it came in "a very short period of time and under difficult and challenging security conditions."

However, experts have not had access to 12 chemical weapons production facilities yet, Kaag said.

She said the timely removal of toxic chemicals had become even more pressing to ensure "none of the chemical weapons material falls in the wrong hands," referring to rebels trying to overthrow Assad who include the increasingly influential al-Qaida-linked Nusra Front.

She said fighting in areas where sensitive sites were located could rapidly deteriorate, making a "timely and swift extraction even more" important.

Syrian officials did not comment on Kaag's remarks.

The international community aims to remove and destroy 1,300 metric tons of chemicals Syria stockpiled to turn into poison gas and nerve agents.

The effort was sparked by an Aug. 21 chemical weapons attack near Damascus that killed hundreds of people. The attacks were blamed on Assad's government and brought the United States to the brink of military intervention in Syria. Damascus denied involvement.

In recent weeks, activists have accused government forces of attacking rebel-held areas with poisonous chlorine gas, according to Associated Press interviews with more than a dozen activists, medics and residents on the opposition side.

Syria denies the allegations, and they haven't been confirmed by any foreign country or international organization.

But if true, they highlight the limitations of the global effort to rid Syria of toxic material.

Meanwhile, Sawsan Omar Haddad, a 51-year-old engineer from the coastal province of Latakia, became the first woman to register as a candidate in Syria's upcoming June 3 presidential election.

In a state television broadcast Sunday, parliament speaker Jihad Laham said Haddad had registered her candidacy a day earlier.

Another three candidates also registered Sunday, bringing the total number of contenders to six.

The other three who registered Sunday were Samir Ahmed Moalla, a 43-year professor of international law from the southern province of Quinetra; Mohammad Firas Rajjouh, 48, from Damascus; and Abdul-Salam Salameh, 43, from the central province of Homs.

Assad has suggested he would seek a third, seven-year term, though he has not announced his candidacy yet.

Analysts said they expected at least one candidate to run against Assad to give the election a veneer of legitimacy.

Syria's opposition have blasted the decision to hold presidential elections amid the country's 3-year-old conflict, which has killed more than 150,000 people and driven a third of the country's population from their homes.

Syria's foreign ministry rejected the criticism, saying the decision to hold presidential elections was "sovereign." It warned that "no foreign power will be allowed to intervene" in the process.

Also Sunday, rebels blew up the city's industrial chamber in a blast that rocked the northern city of Aleppo, said state-run television and activists.

The bombing was part of a surge by rebels in Aleppo to push back against government attempts to take opposition-controlled parts of Syria's largest city.

The fight for Aleppo is particularly important now, with analysts saying they expect Assad's forces will try wrest as much of the city as possible before elections.

The explosion killed at least six people, said state-run television. Lebanese channel al-Mayadeen, that closely covers Syria, reported 25 dead. Conflicting death tolls are routine after large bombings.

Rebels dug a tunnel to reach the chamber in government-controlled territory, said Aleppo activist Hassoun Abu Faisal. The building was used by pro-government forces, reported the Britain-based Syrian Observatory for Human Rights.

Activist Abu Faisal said ultraconservative Sunni fighters had organized a joint campaign since February to push back against pro-Assad forces, who had been encroaching on rebel supply lines in eastern Aleppo.

There are now three active fighting fronts in Aleppo, reported al-Mayadeen, with some areas now changing hands on a daily basis.

Abu Faisal and other activists in Aleppo have said weeks of intensified clashes have also included the government increasing its shelling and bombing of rebel-held areas.

The Syrian Observatory, which obtains its information from activists on the ground, said at least 70 people were either killed or wounded in Sunday's fighting throughout Aleppo, which also included government aircraft shelling residential rebel-held areas.

Surk reported from Beirut. Associated Press writers Yasmin Saker and Diaa Hadid in Beirut contributed to this report.