Secretary of State George P. Shultz announced yesterday that U.S. troubleshooters Philip C. Habib and Morris Draper are undertaking another Middle East mission this week but there was no indication of a break in the logjam over the withdrawal of Syrian forces from Lebanon.

"We continue to be fully engaged" in efforts to bring about the withdrawal of all foreign forces from Lebanon, Shultz told a news conference. He conceded, though, that "the problems are real," especially Syria's refusal under present circumstances to pull out its troops.

Syria declined in mid-May to accept a visit from Habib, a refusal that stands as a limiting factor on the veteran diplomat's usefulness in the current situation. Because of the Syrian position, Habib "won't go" to Damascus, Shultz said, "and we will find other people who we hope will be acceptable to them."

The secretary gave no hint who the substitute emissary might be. Another official said planning is "not far along" on a new approach to the Syrians.

Shultz, who leaves today on a two-week swing to the Philippines, Thailand, India and Pakistan, did not foreclose the possibility that he will visit Damascus near the end of his trip. However, he said that he currently has no plan to do so.

Earlier this week there were reports that Habib was reluctant to continue his often-grueling task of negotiating in the Middle East, a job for which President Reagan called him out of retirement in 1981. Officials said Habib's decision to return to the area was made during a State Department policy review that was completed Tuesday night.

In striving for the withdrawal of all foreign forces from Lebanon, the administration has discouraged a partial Israeli withdrawal out of concern that it would make a complete withdrawal less likely. Whether such a partial Israeli withdrawal would be a setback, Shultz said yesterday, "depends on the conditions under which it takes place," a remark that appeared to suggest a greater U.S. willingness to contemplate such action by Israel.

Regarding relations with the Soviet Union, Shultz said his intention in a 35-page policy statement to the Senate Foreign Relations Committee last Wednesday had been to show "both sides of the coin"--enhanced U.S. strength and willingness to engage in diplomacy.

"We want to make it clear we are determined to be strong, to be able to defend our interests," he said. "We have strong allies. We are working effectively with them. There is cohesion. We are determined.

"At the same time, we and our allies both would prefer a more constructive dialogue and set of arrangements with the Soviet Union than we have had. In the statement I said that we are prepared to engage in that kind of discussion."

Shultz repeated the view he expressed last week that "substantive results" in diplomacy are crucial to any improvement in Soviet-American relations. Soviet Foreign Minister Andrei Gromyko, in a statement in Moscow Tuesday, took a similar stand.

At one point Shultz remarked that "there are some areas where there has been some progress made" in Soviet-American discussions, which are taking place in a variety of negotiations and contact points throughout the world.

He went on to say, however, that "barring substantive results that we can point to that are significant, I don't think you could say the process is leading us somewhere.

"There's a lot of input, there's a lot of activity, but we have to keep looking at the substance and asking ourselves in those terms whether or not we're getting anywhere, and so far we're not," Shultz said. Nevertheless, he said he will continue trying.

Shultz said it is "a little bit difficult" to determine whether progress is being made in the United Nations-sponsored talks in Geneva about a possible political solution in Afghanistan leading to the withdrawal of Soviet military forces. The secretary said he hopes to learn more from Pakistani officials, who have been key participants.

Regarding Central America, Shultz condemned "very large-scale shipment" of Soviet arms into Nicaragua, directly and through Cuba, which he said "is not appreciated by us" and is "a very unfriendly thing to do."

He declined to give a specific assurance that the United States would step in on the side of Honduras if that country were attacked by Nicaragua, but declared that "we would take a major outbreak of war there very seriously."

Earlier this month, Gen. Gustavo Alvarez Martinez, chief of the Honduran armed forces, asked for a U.S. commitment to use its military power, including troops, in case of a major invasion from Nicaragua.