The children's faces light up when they see the contents of Osborne's bag and they lose no time in experimenting with the triangles, wooden claves, maracas and Tibetan bells. Osborne soon has the children beating out catchy samba rhythms and singing traditional Sevdalinkas (archetypal multi-cultural songs influenced by Dalmatian, Ottoman and Hungarian music). Such is the youngsters' intense concentration and enjoyment that no one notices the noise from a nearby explosion. Only days later, another shell is to land not far from the cellar, severely injuring a number of children from the local kindergarten. More often than not, it is children who are on the receiving end of these sporadic Serb attacks.
Until recently, Mostar, located in the south-west of Bosnia, enjoyed a fragile peace, but current Serbian army advances in the Eastern enclaves have also led to an escalation of shelling activity in the city. This has created an undercurrent of fear in a population that was subjected to constant bombardment during the early years of the war; first by Serb forces, then by Croatian nationalists who pounded east Mostar almost to complete rubble and deliberately destroyed the 400-year-old Stari Most bridge.
In 1994, the installation of the EU as administrators of Mostar and an agreement between the Croatian and Bosnian governments to form the Federation of Bosnia and Herzegovina brought about a tentative peace. But despite the Bosnian government's wish to re-unify the city, Mostar is still divided by an invisible Berlin Wall into east and west, with little freedom of movement between the two sides.
However, the window of opportunity gave Osborne and others the chance to begin much-needed creative education work with the children in the city. In May, Osborne, together with pop musician and artist Brian Eno, visited Mostar to run a series of workshops and to step up plans for a new music centre in the city. The charity, War Child, of which Eno is a patron and Osborne the music adviser, is looking to raise pounds 4m to transform the rubble of a former Austro-Hungarian primary school into a valuable community resource.
The music centre would include performance areas, teaching rooms and a commercial recording studio in the basement where it's hoped that artists like Eno will come and work. But the most important aspect of the centre will be a special music- therapy wing devoted entirely to children traumatised by the war.
During their visit, Eno and Osborne began this much needed healing process with an outdoor workshop in the grounds of the shelled primary school. The children eagerly banged, bashed and shook a variety of exotic percussion instruments brought by Eno in a large black box. Most of the city's instruments were destroyed during the war, including pianos that were burned for fuel. However, one boy managed to salvage a shell-damaged drum patched up with Sellotape from the local theatre, which produced a dull thudding noise.
The children were also keen to perform in the dank cellars of the former school where many of them took shelter during the worst moments of the war. Here they proudly played their own composition, The River Neretva, evoking the sound of the peppermint-green river that cascades through the centre of Mostar.
As well as his creative input to the music centre project, Eno and his wife, Anthea, have also organised a number of successful fundraising events. Luciano Pavarotti has offered to donate the proceeds from his concert in Modena, Italy, on 12 September, to the music centre. Members of U2, along with other big names from the musical world, will also be appearing.
In the meantime, Osborne is continuing to visit Mostar to work with different groups including children from the Blind Institute in West Mostar and gypsy refugees. On this latest visit, he was joined by Andrew Kerr, the Music Adviser for Lothian Region in Scotland. Together, they visited all the music clubs and groups in east Mostar to draw up a list of requirements.
Osborne says the rebirth of cultural life and education is happening in several parts of the city, through little clubs and other group initiatives fostering creative and artistic work. "Even if there is a lack of specialist knowledge and equipment, there is so much enthusiasm and ability among the people that the ground is very fertile and ready to be redeveloped. Many of the groups have continued their activities, despite the war situation, meeting in the relative safety of underground cellars."
These include the Camarad Music and Folk Dance Group, Jelka's Music and Painting Club, Apeiron, who are interested in fine arts, installations, mixed media and literature, and Suad's Club, who are planning a photography course alongside teaching music and keyboard skills. "It's interesting to see that music is the common thread running through this cultural regeneration process," says Osborne, "and I think the musical world has a particular duty and role to play in this revival."
For Osborne, the Mostar project is the latest example of his tireless commitment to cultural life in Bosnia. In 1992, he played in the front line in Sarajevo with Bosnian cellist Vedran Smailovic. Last summer his opera, Sarajevo was performed at the South Bank in London, and in February he wrote the opera Europe, which opened the Sarajevo Winter Festival. He has also run several children's projects in Sarajevo.
Some might argue that providing food for Bosnians is more important than musical initiatives. However, Osborne maintains that with much of the humanitarian effort failing, cultural initiatives, which are very highly valued by local communities, can have a high success rate: "While other aid is a double-edged weapon, there is no way anyone can demand 50 per cent of what I do. A human resource cannot be divided. This work is my personal commitment to what is perhaps the most valuable key to society in Europe, to a multi-cultural society of high cultural standards and interests. Culture is an important glue for Bosnian people, putting something positive, creative and visionary in a place where there is a lot of despair and suffering, and supporting optimism in a context of mayhem."
n War Child, 7-12 Greenland Street, London NW1 (0171-916 9276). Next War Child fund-raiser is a concert by Steeleye Span at Forum, London, NW5 on 2 Sept (0171-284 2200)