* Election marred by allegations of fraud
* Final results due on July 22
* US warns "extra-legal" measures will cost Afghanistan US support (Adds US statement, paragraphs 6-7)
By Mirwais Harooni and Maria Golovnina
KABUL, July 8 (Reuters) - Afghan presidential contender Abdullah Abdullah's camp rejected preliminary results of last month's run-off election on Monday as a "coup" against the people, putting him on a dangerous collision course with his rival, Ashraf Ghani.
The Independent Election Commission announced on Monday that Ghani won the June 14 second round with 56.44 percent of the vote, according to preliminary results. The tally might change when the final official numbers come out on July 22.
Abdullah's camp responded angrily, saying the result was invalid because it did not throw out all the fraudulent votes.
"We don't accept the results which were announced today and we consider this as a coup against people's votes," said Mujib Rahman Rahimi, a spokesman for Abdullah's campaign.
His rejection sets the stage for a possible bloody standoff between ethnic groups or even secession of parts of the fragile country, which is already deeply divided along tribal lines.
In a blunt warning, U.S. Secretary of State John Kerry said Washington expected "a full and thorough review of all reasonable allegations of irregularities" and that there was no justification for violence or "extra-constitutional measures".
"Any action to take power by extra-legal means will cost Afghanistan the financial and security support of the United States and the international community," Kerry said in a statement issued as he travelled to China on Tuesday.
Abdullah, a former anti-Taliban resistance fighter, has long alleged widespread fraud in the messy and protracted vote and insisted results should be delayed until all problematic poll stations have been audited.
Son of a Pashtun father and a Tajik mother, he draws much of his support from the Tajik minority in northern Afghanistan and is capable of drawing massive crowds who are likely to be equally enraged by Monday's announcement.
Ghani, for his part, has strong support from Pashtun tribes in the south and east. Hundreds of people took to the streets to celebrate in the southern city of Kandahar late on Monday.
However, officials warned this was not the final result.
"The announcement of preliminary results does not mean that the leading candidate is the winner and there is the possibility the outcome might change after we inspect complaints," IEC chief Ahmad Yousuf Nuristani told reporters, a view echoed in an earlier statement from the U.S. State Department.
COMPROMISE OR DEADLOCK?
The deadlock over the vote has quashed hopes for a smooth transition of power in Afghanistan, a concern for the West as most U.S.-led forces withdraw from the country this year.
Earlier on Monday, rival camps tried to find a last-minute compromise to keep Afghanistan from sliding into a protracted period of uncertainty.
Nuristani said the commission had received a request from Abdullah's camp to review ballot papers from more than 7,000 polling stations on suspicion of fraud, which could significantly alter the result if recounted.
"We announced preliminary results today and it is now the complaints commission's duty to inspect this case," he said. "We are ready to provide any assistance until the end of the process."
The vote to pick a successor to Hamid Karzai was intended to mark the first democratic transfer of power in Afghan history, a crucial step towards stability as NATO prepares to withdraw the bulk of its troops by the end of the year.
Western powers, particularly the United States, had hoped for a trouble-free process that would show that 12 years of their military involvement in Afghanistan were not in vain and contributed to the country's nation-building.
But the process has been fraught with accusations of cheating from the start.
Without a unifying leader accepted by all sides, Afghanistan could split into two or more fiefdoms along tribal fault lines, or even return to the bloody civil war of the 1990s.
Abdullah has accused Karzai, also a Pashtun, of playing a role in the alleged rigging in Ghani's favour and says he would accept the vote only if he saw firm evidence that fraudulent votes had been thrown out and the final result was clean.
Taliban insurgents remain a formidable security risk after vowing to disrupt the election process. On Monday, they killed a district police chief in the western city of Herat and attacked a check point in northern Afghanistan. (Writing by Maria Golovnina; Editing by Paul Tait and Matt Driskill)