The Swiss transport minister, Moritz Leuenberger, and his German counterpart, Kurt Bodewig, have failed to iron out differences in a long-standing dispute over noise pollution out of Switzerland's main airport of Zurich.
Germany reiterated that it would take unilateral steps if no agreement were reached at the final meeting in Bonn next month.
Addressing a news conference in Bern on Tuesday, both ministers said they narrowed their differences in the latest round of talks. However, three issues still remained to be resolved.
Leuenberger said no consensus was reached on demands for financial compensation by German towns and villages. They have complained about noise and environmental pollution out of Switzerland's main airport of Zurich.
Differences also remain over a timetable for the gradual reduction in the number of flights using air space over southern Germany before landing at Zurich airport.
In addition, both sides are at odds over whether the air transport treaty with Germany is compatible with an accord, which is pending, between Switzerland and the European Union.
Bodewig reiterated on Tuesday that Berlin would unilaterally limit the number of flights permitted in German air space to 80,000 in 2003, if no final accord is reached with Switzerland next month
In response, André Auer, director of the Federal Office for Civil Aviation, was cautiously optimistic that an agreement can be reached.
"We certainly have enough elements to analyse the situation and to try to bridge the gap. We still hope that it will be possible to find a mutual agreement", he told swissinfo.
In April Leuenberger and Bodewig agreed that Switzerland would reduce the number of flights using German airspace when landing at Zurich from currently about 150,000, to just below 100,000 by the year 2005.
Under the deal, night flights over German territory will be banned from next month, and a reduction in flights on weekends will be phased in within a year.
The controversy over an air transport treaty with Germany has sparked a heated debate in Switzerland. The government is facing criticism from the authorities at Zurich airport, local councils and some of the main political parties.
Some critics accused Leuenberger of being too soft on German demands and of jeopardising the competitiveness of Zurich airport. Leuenberger has repeatedly defended his policy. He said certain concessions were necessary to avoid an escalation of the conflict.
swissinfo with agencies