The remarks come as the U.S. debates whether to substantially increase its forces in Afghanistan or to conduct a more limited campaign focused on targeting al Qaeda figures - most of whom are believed to be in neighboring Pakistan.
The minister's comments hit on a key worry of the United States - that not sending enough troops to Afghanistan will open the door back up to al Qaeda. They also suggest that the Afghan government is nervous about the U.S. commitment amid talk of changing the strategy and a surge in violence in recent months.
An American and two Polish troops were killed by bombs in the latest violence reported by NATO forces.
"The enemy has changed. Their number has increased," Defense Minister Gen. Abdul Rahim Wardak told lawmakers in a speech. He said about 4,000 fighters, mostly from Chechnya, North Africa and Pakistan "have joined with them and they are involved in the fighting in Afghanistan."
He gave no timeframe for the supposed increase in foreign fighters.
Wardak said Afghan intelligence services had asked for more international forces to cope with the foreign threat, and the minister's spokesman said Wardak backed the call.
U.S. military officials said they could not immediately comment on the claim of a recent influx of foreign fighters.
Afghanistan's interior minister, who also spoke to parliament, endorsed a strategy promoted by the top U.S. commander Gen. Stanley McChrystal to focus on protecting civilians rather than simply killing insurgents.
"If the target of this fight is only killing the Taliban, we will not win this war. If it is saving the Afghan people, then we have a possibility," Interior Minister Hanif Atmar said.
The strategy debate in the U.S. has been complicated by the still-undecided Afghan presidential election, which has raised doubts about whether there will be reliable, credible Afghan leadership to cement any military gains by the U.S. and its allies. Results from the disputed August vote have been delayed because of widespread allegations of fraud.
A U.N.-backed fraud investigation panel was analyzing data Saturday from an audit and recount of polling stations with suspect results. Results from about 13 percent of the country's polling stations hang in the balance - enough to swing the result from an outright win by President Hamid Karzai to a forced runoff between the top two finishers.
Election officials have said they expect to announce final results by the end of next week.
The weeks of waiting have been dogged by accusations of wrongdoing between candidates and even within the U.N., which has advised on the vote and whose appointees dominate the fraud investigation panel.
The second-in-command at the U.N. in Afghanistan was fired this month after a dispute with his boss about how to investigate alleged fraud. The official, American Peter Galbraith, has since accused the U.N. of trying to cover up fraud to avoid a runoff vote. Kai Eide, the top U.N. envoy to Afghanistan, has denied the charges.
An Afghan election official said Saturday that the U.N. dispute is further damaging the credibility of an election already seen as marred by fraud.
"This kind of controversy will of course have an impact on the wider perception of the election inside and outside Afghanistan," said Zekria Barakzai, a deputy chief electoral officer with the government-appointed Independent Election Commission. "It is a negative impact."
The U.S. service member died Saturday of wounds suffered in a bombing in southern Afghanistan, U.S. officials said.
The Polish soldiers were killed when their vehicle hit a roadside bomb in eastern Wardak province on Friday, Poland's Defense Ministry said. Four others were wounded.
Also Saturday, Afghan officials said nine Taliban were killed the day before in a gunfight in Wardak.
Two Afghan policemen and a district official were killed Friday when their vehicle struck a roadside bomb in Paktika province, which borders Pakistan.