Afghan President Hamid Karzai's top challenger called for a "dramatic increase" in troops to ensure security in his country, suggesting a failure to send in reinforcements could put Afghanistan "at risk" of falling to insurgents. 

"The need for more troops is there in order to reverse the situation," Abdullah Abdullah, who is expected to face off against Karzai in a runoff election in two weeks, told "Fox News Sunday." 

President Obama is deliberating over a request from his top commander for such an increase. He's weighing a call by Gen. Stanley McChrystal and other military officials for 40,000 more troops against calls to pursue a more surgical strategy aimed at top Al Qaeda leaders, particularly in Pakistan. 

But Abdullah did not say whether Obama is taking too long to determine whether to grant his top commander's request for more forces. 

"Even if the decision is made today it doesn't mean tomorrow we'll have troops on the ground. It will take time," he said. But he suggested that as long as Karzai is in power, the war effort will be hampered. 

"The Afghan side has not been able to deliver, and the Afghan side has been led by Mr. Karzai," Abdullah said. 

A runoff was ordered following findings of widespread fraud in the August election, much of which tilted in Karzai's favor. Abdullah told "Fox News Sunday" that he was not inclined to consent to a power-sharing agreement to avoid the second round.

"I think I should rule it out, because I'm ready to go for a runoff," he said. 

But he still warned about the possibility that the runoff will bring with it even more fraud if key reforms are not made. Abdullah reportedly wants the leadership of the election commission replaced before the vote. 

"Perhaps we might have to go through the same sort of saga," he said. "It will make the situation very difficult if those conditions ... are not met." 

Abdullah warned that without assurances that the vote will be credible it will be difficult to convince voters to turn out. He did not say whether he would consider a boycott of the runoff if reforms are not made.