Afghan presidential candidate Abdullah Abdullah on Tuesday said he received calls from President Barack Obama and U.S. Secretary of State John Kerry after he refused to accept the preliminary result of the vote citing fraud.

Abdullah told thousands of supporters at a gathering in Kabul that Kerry would be flying to the Afghan capital this Friday for meetings and help defuse the crisis.

The turmoil came as violence escalated around the country.

An Afghan official said that at least 16 people, including four Czech soldiers, were killed Tuesday in a suicide attack near a clinic in eastern Afghanistan.

Suicide bomber

The Czech Ministry of Defence said Tuesday four Czech troops were killed and another was badly wounded after the blast. The ministry said it will release more details later in the day.

Wahid Seddiqi, spokesman for the provincial governor of Parwan province said the soldiers, at least 10 civilians, and two police officers were killed when a suicide bomber attacked Afghan and foreign forces near Charakar, the provincial capital.

The Taliban claimed responsibility for the attack in a statement sent to the media

U.S. State Department officials accompanying Kerry in Beijing declined to comment on his travel plans.

Abdullah told his supporters that the results of the election were fraudulent, but asked them to give him a few more days to negotiate.

"We denounce and do not accept the results of fraudulent vote. I assure you people of Afghanistan that I will sacrifice for you, but I will never accept a fraudulent government," he told his supporters, many angry over the result. "We announce that only the government elected through clean votes will come to power."

Turnout was 50%

The Afghan Independent Election Commission released preliminary election results Monday showing former finance minister Ashraf Ghani Ahmadzai well in the lead for the presidency but said no winner could be declared because millions of ballots were being audited for fraud.

Preliminary results announced Monday showed that Ahmadzai had about 4.5 million votes, or 56 per cent, while Abdullah had 3.5 million votes, or 44 per cent, according to the commission. Turnout was more than 50 per cent.

The election commission has acknowledged that vote rigging had occurred and said ballots from about 7,000 more of the nearly 23,000 polling stations would be audited.

Abdullah charged that outgoing President Hamid Karzai, Ahmadzai and the election commission were colluding.

"President Karzai, the election commissions and Ashraf Ghani's team are all one. They ignored us and announced the fraudulent results," he said.

There were fears that Abdullah could ignore the result and declare victory, something he hinted at during his speech.

End to U.S. support

"People across the county call on us to announce our government, and I can't not say no to people's wish, we don't want civil war, we don't want crisis. All of our lives we defended this country. We do not want crisis, we want national unity," Abdullah said. "We don't want separation, we don't want civil war."

U.S. Secretary of State John Kerry said during a visit to Tokyo that any action to seize power illegally in Afghanistan would lead to the end of U.S. financial and security support.

Kerry said suggestions of a "parallel government" in Afghanistan were a grave concern and added that he expected Afghan electoral institutions to conduct a full review of all reasonable allegations of irregularities. He said there was no justification for violence or threats of extralegal action.

"President Obama called me today saying John Kerry is coming to Kabul on Friday. President Obama and Kerry both called me this morning, promised to help us in cleaning up votes," Abdullah told the crowd. "John Kerry phoned me before the announcement and told me that his envoys would seek a legal solution for the problems created in here."