Israel hardened its bargaining position today in the negotiations for a troop withdrawal from Lebanon.

A Foreign Ministry spokesman said reports that Israel was backing down on its demand that the remaining Palestine Liberation Organization guerrillas leave Lebanon before Israeli forces withdraw were "without foundation."

"Israel is continuing to insist that the terrorists be the first to leave all of Lebanon," the spokesman said.

The statement, issued with the approval of top government officials here, contradicted the position voiced earlier this week by Foreign Minister Yitzhak Shamir at the United Nations.

In an interview, Shamir had said Israel was willing to consider a simultaneous withdrawal of Israeli, Syrian and Palestinian forces. He also said Israel's call for the PLO forces to leave Lebanon before the Israelis or the Syrians was not a firm demand but was a subject for negotiations.

Such statements gave rise to considerable confusion regarding the Israeli negotiating position that may have reflected some uncertainty within the government of Prime Minister Menachem Begin. They also led to reports in the Israeli press that Israel had bowed to a demand by the United States that it not complicate the troop withdrawal negotiations by insisting on conditions such as that the PLO guerrillas withdraw first.

Today's statement appeared to represent a victory for the hard-liners in the Begin government, including Defense Minister Ariel Sharon, whose goal remains the removal of all Palestinian forces from Lebanon.

When U.S. envoy Morris Draper discussed the troop withdrawal with Begin, Sharon and Foreign Ministry officials earlier this week, Israeli officials said that there were "no differences of opinion." However, they conceded that the subject of how the withdrawal should be timed did not come up during Draper's talks here.

American officials have stressed the importance of arranging a rapid troop withdrawal to prevent renewed fighting that could erupt into warfare along the line that separates Israeli troops from the Syrian and Palestinian forces in central and eastern Lebanon.

Israel has said repeatedly that it holds Syria responsible for PLO activity in Lebanon and it has replied to cease-fire violations in the area by attacking Syrian targets. The Israelis thus want the PLO withdrawal to occur while their own forces are still in a position to attack the Syrians inside Lebanon.

For its part, Syria is insisting that the Israelis be the first to pull back from their present positions, if not to withdraw completely from Lebanon.

Even in the event of a withdrawal agreement, it remains unclear how the Israelis intend to verify that the PLO forces, which Israel estimates at 4,000 to 5,000 guerrillas stretched between the northern port city of Tripoli to the Bekaa Valley in the east, have left Lebanon. Asked about this today, officials here referred vaguely to the desire of the Lebanese government to see the Palestinians gone.

Suggesting that this task could be handled by the Lebanese Army, an official said, "since we are not the only ones who want them out we hope that things will work better" than in the Beirut evacuation.

The timing of a withdrawal is one major stumbling block to a quick agreement. Another is the status of southern Lebanon, where, Sharon emphasized yesterday, Israel remains determined to establish a 25-mile-wide "security belt" to protect its northern border.

Sharon toured the area yesterday and met with former Lebanese Army major Saad Haddad, the commander of an Israeli-backed Christian militia in southern Lebanon, in a show of continued Israeli support. The defense minister said Israel will not turn over control of the area to any other force unless it obtains a "signed agreement" on security arrangements with the Lebanese government.

Sharon did not insist, as he has in the past, on a peace treaty between the two countries. But the new government of Lebanese President Amin Gemayel is keeping a great distance from Israel, making the prospects of any kind of formal pact between the countries in the near future appear doubtful.

In another development today, Israeli officials, stressing that they will not ask the United States to share the cost of the war in Lebanon, presented U.S. Ambassador Samuel W. Lewis with an analysis of their military and economic aid needs for the fiscal year that begins Oct. 1, 1983. The analysis asks for $1.96 billion in military aid and $1.22 billion in economic aid, a total of $3.18 billion.

For the current fiscal year, Israel had asked for $3.08 billion in military and economic aid. This was cut to $2.48 billion by the Reagan administration in its request to Congress, but the amount has not been approved because the foreign assistance appropriation bill remains stalled on Capitol Hill.