KABUL, Afghanistan (AP) -- The European Union called on Thursday for a more extensive investigation into allegations of irregularities in Afghanistan’s presidential election, citing "highly worrying indications of potentially widespread fraud."

The statement came a day after the Afghan commission overseeing the vote postponed the release of preliminary results until next week so it could audit the ballots from 1,930 polling stations that had at least 599 votes in 30 different provinces.

Thijs Berman, the head of the EU’s election monitoring team in Kabul, welcomed that step but said the commission’s choice to audit "only polling stations with 599 votes and over significantly limits the possible detection of fraud."

Other factors also should be examined, including highly improbable votes for one single candidate in polling stations, or unlikely discrepancies between votes cast by women and men he added.

Altogether, the number of problematic polling stations could well exceed 6,000 of the 22,828 stations nationwide, Berman said at a news conference in Kabul.

"An additional in-depth audit of the votes is necessary, given these highly worrying indications of potentially widespread fraud," he said. "This is technically possible without much delay."

Abdullah Abdullah, a former foreign minister and one-time aide to a famed warlord during the Afghan anti-Soviet guerrilla campaign, garnered the most votes in the first round of voting on April 5 but failed to get the majority needed to win outright. He has alleged widespread ballot box stuffing and other efforts to rig the June 14 runoff vote in favor of his rival Ashraf Ghani Ahmadzai, a former Finance Minister and World Bank official.

Ahmadzai’s team also has registered complaints of fraud but called for the Independent Election Commission’s process to be respected.

Whoever wins will replace President Hamid Karzai, the only leader the country has known since the 2001 U.S.-led invasion that ousted the Taliban. He was constitutionally barred from seeking a third term.

Western officials had hoped the election would be an important step toward democracy for the troubled country as the U.S. and its allies wind down their 13-year combat mission. Both candidates have promised to sign a security pact with the United States that would allow nearly 10,000 American troops to stay in the country beyond the end of this year to train Afghan security forces and perform counterterrorism operations.

Karzai’s 2009 re-election also was marred by widespread ballot box stuffing and proxy voting, leading Abdullah, who was runner-up at the time, to refuse to participate in the runoff.

The U.N. mission in Afghanistan also welcomed the IEC’s decision to delay the announcement of preliminary results until Monday and encouraged the panel "to implement further measures that could enhance the transparency, neutrality and impartiality of the electoral process, and separate and reject fraudulent ballots from valid votes."

Insurgents, meanwhile, have stepped up attacks as part of their annual summer offensive when they take advantage of warmer weather to move more freely in the mountainous country.

Taliban fighters fired two rockets into the military side of the Kabul airport on Thursday, striking Karzai’s helicopter as it sat empty on the tarmac, an official said.

Abdul Wahab Wardak, the commander of the military airport, said only one of the two rockets exploded and no casualties were reported. But he said the Russian-made military helicopter used to transport Karzai was set on fire.

The attack on the airport, which is in one of the most heavily guarded areas of the Afghan capital, underscores the resiliency of militants led by the Taliban who are fighting against the Western-backed government.

A bomb hidden inside a garbage bin exploded in the northern city of Mazar-i-Sharif, killing one person and wounding five others, police spokesman Shir Jan Durani said.

And a land mine struck children were playing on a field in the western Herat province, killing a girl and a boy and wounding eight other children, provincial chief of police Abdul Raouf Ahmadi said.

The U.N., meanwhile, said the number of child casualties caused by the conflict in Afghanistan rose by 30 percent in 2013 compared with the previous year, with at least 545 children killed and 1,149 wounded.

Citing the U.N. secretary-general’s annual report on Children and Armed Conflict, it also expressed concern about the continued recruitment and use of children by insurgents as well as Afghan government forces. The Taliban and other armed opposition groups were responsible for 72 of the 97 cases recorded last year, including nine boys recruited to conduct suicide attacks, the report found.