Media playback is unsupported on your device Media caption Abdullah Abdullah: "Without doubt we are the winner of this election"

Afghan presidential contender Abdullah Abdullah has claimed victory in last month's poll, despite results giving a lead to his rival, Ashraf Ghani.

Addressing supporters in the capital, Kabul, Mr Abdullah repeated claims that the election was marred by fraud.

US Secretary of State John Kerry had earlier warned against a power grab, amid reports that Mr Abdullah was planning a "parallel government".

Meanwhile, a bomb near Kabul has killed 16 people, including four Nato troops.

Ten civilians and two police officers were also reportedly killed in the attack on a clinic near Bagram, home to the largest US base in the country.

At the scene: BBC correspondent Harun Najafizada, Kabul

Media playback is unsupported on your device Media caption Karen Allen at pro-Abdullah rally: Supporters pulled down a Hamid Karzai poster

People at the venue where Abdullah Abdullah spoke to his supporters were angry. A crowd tore down a poster of outgoing President Hamid Karzai chanting: "Death to Karzai. Long Live Abdullah!"

They are appalled at the election results. They believe that massive fraud has happened while President Karzai, the election commission and Ashraf Ghani stood by.

So the Abdullah camp have decided to take things into their own hands and he appears to have some support. Several influential figures - governors, district leaders, mayors, and warlords - have come out in his favour. But Afghanistan is a divided country - Mr Ghani and Mr Abdullah both command a lot of support.

Analysts will be very worried. They know that what has been suggested - a parallel government - could affect Afghanistan's stability, economy, security and its relations with the international community. The army across the country is such a divided force that it is difficult to know how it might respond. Everybody recognises this has the making of an electoral crisis.

Palpable rage among Abdullah supporters

Mr Abdullah told the gathering in Kabul that he would never "accept a fraudulent government".

"We are the winners of this round of elections, without any doubt," he said, to cheers from the crowd.

However, he stopped short of declaring a parallel government as his supporters had earlier suggested he might. Instead, he called for a unified country.

"We don't want civil war, we don't want a crisis," he said. "We want stability, national unity, not division."

Image copyright AFP Image caption Ashraf Ghani (centre) called on his opponent to not "drag the country into further crises"

Preliminary results announced on Monday gave Mr Ghani 56.44% of votes in the 14 June run-off.

Mr Abdullah, who fell just short of an outright majority in the first round, had 43.56%.

Both men have alleged fraud in the election. Votes are being re-checked at more than 7,000 polling stations - nearly a third of the total number.

Correspondents say recounts could significantly alter the final result, due on 22 July.

Mr Ghani welcomed Monday's results as "legitimate and credible", but added that he was not afraid of a vote recount "for the sake of transparency".

"Our votes are clean," he said at a press briefing in Kabul on Tuesday.

Image copyright AFP Image caption Nato troops based at Bagram have been fighting Taliban insurgents

The United Nations Assistance Mission to Afghanistan warned it would be "premature" for either side to claim victory.

It called on the presidential candidates to exercise restraint and prevent their supporters from "making any irresponsible statements and from taking steps that could lead to civil disorder and instability".

Earlier on Tuesday, Mr Kerry said Afghanistan risked losing security and aid support if anyone tried to "take power by extra-legal means".

Mr Abdullah said Mr Kerry was expected to travel to Afghanistan on Friday. US officials have not commented on the secretary of state's travel plans.