A second mortar hit, slightly closer. It nearly blasted out the windows of the military barracks' office and shook a cloud of dust off the plastic flowers on Mr Massoud's desk. This time, he glanced around with a look of annoyance. He turned to his guests: 'Would you feel more comfortable in a safer place?' I suggested he was a better judge of the danger than we were.
Before he could reply, a third mortar thudded in. This one landed just as near. But while the other two shells had thumped in behind us, this last one fell in front of the building. In military parlance, we were being 'bracketed'.
Mr Massoud sprang to his feet and ordered us to follow him to an underground bunker. There, he explained why Kabul's fate depended on who controlled this valley of Kohistan, 45 miles (72km) from the capital.
He explained how guerrillas led by a stern, rebel fundamentalist, Gulbuddin Hekmatyar - who refuses to join the Islamic government in Kabul even though he was recently named its prime minister - had blockaded the southern and eastern roads into Kabul, effectively cutting off all direct supply routes between the capital and Pakistan.
Kabul's only lifeline is to the north, the Salang Highway, linking it to the Central Asian republics. The highway passes through Kohistan valley, which is under the control of Mr Hekmatyar's most powerful commander, Farid.
For more than two weeks now, Mr Massoud's government forces, backed by more than 40 tanks, have hammered Commander Farid's positions in this valley. A contingent of Massoud loyalists out-flanked Commander Farid and have cut off his retreat to the Kabul-Peshawar road.
In a desperate, last attempt, Commander Farid sent a platoon of men carrying a few mortars to scale the towering ridge behind Mr Massoud's headquarters. His guerrillas succeeded in this mission and were hurling down shells with unnerving precision when my colleagues and I visited Mr Massoud.
Even if Mr Massoud defeats Commander Farid, as he is expected to within a week, Kabul is far from safe. Mr Hekmatyar and his allies, the Shia Muslim armies, backed by Iran, control the city's southern suburbs and are within heavy artillery range. Every day they pound Kabul with rockets.
Mr Masoud's forces retaliate, killing an equal number of people in the Shia neighbourhoods. More than 6,000 people were killed and 20,000 injured in the past year of internecine fighting since the fall in April 1992 of the pro-Soviet regime of Najibullah.
Leaders of the nine mujahedin factions that won the Islamic 'holy war' against the Soviet Union are meeting in Jalalabad, but no lasting ceasefire is expected. Neither Mr Hekmatyar nor Mr Massoud trusts the other enough to share power. Despite cajoling and cash offers from Pakistan, Saudi Arabia and Iran, nobody has the strength to impose peace between them.
Over the past few days, Mr Hekmatyar's forces have been strengthening their positions on the southern and eastern ridges around Kabul, while Mr Massoud is hastening to finish off Commander Farid in the north so he can rush his tanks and men back to Kabul for the next big rebel seige.
Neither the government forces nor Mr Hekmatyar and his allies are strong enough to defeat each other. Mr Massoud claims he will have support from the powerful Uzbek general, Abdul Rashid Dostam, whose militia has seized a swath of land in north-west Afghanistan and also controls Kabul airport and the eastern flank of the city.
Mr Massoud is a Tajik and Gen Dostum an Uzbek, and both clans have traditionally resented the fact that Kabul has been ruled by the Pathans from the south and central regions of Afghanistan. Mr Hekmatyar is a Pathan.
Speaking in his bunker, Mr Massoud denied his intention was to rule Afghanistan. 'We occupied Kabul first. If I wanted to take power, I could have,' he claimed. 'But I didn't want to cause problems with the other nationalities (the different ethnic clans) and the mujahedin parties. But one person wanted to fight.' He left the name of Hekmatyar unsaid. In this brutal stalemate, it is the battered citizens of Kabul who pay.Reuse content