Military vehicles took to the streets of the Zimbabwean capital and prolonged gunfire was heard near the presidential residence early Wednesday as questions mounted over Robert Mugabe's grip on power, even as the army denied a coup in a state broadcast. Tensions between the 93-year-old leader and the military that has helped prop up his reign have intensified in recent days, with Mugabe's ZANU-PF party on Tuesday accusing army chief General Constantino Chiwenga of "treasonable conduct". The military's actions have been seen as a major challenge to Mugabe, but Major General Sibusiso Moyo went on state television in the early hours of Wednesday to deny the army was targeting the increasingly frail leader. "It is not a military takeover of government," Moyo said, reading a statement. "We wish to assure the nation that his excellency the president... and his family are safe and sound and their security is guaranteed. "We are only targeting criminals around him who are committing crimes... As soon as we have accomplished our mission we expect that the situation will return to normalcy," he added. Chiwenga had earlier demanded that Mugabe, who has ruled the country since independence from Britain in 1980, stop purges of senior party figures, including vice president Emmerson Mnangagwa, who was dismissed last week. ZANU-PF said Chiwenga's stance was "clearly calculated to disturb national peace... and suggests treasonable conduct on his part as this was meant to incite insurrection". Before being ousted, Mnangagwa had clashed repeatedly with Mugabe's wife Grace, 52, who is seen as vying with Mnangagwa to be the next president. - 'Uncertainty' - As the situation deteriorated overnight, the US embassy in Harare warned its citizens in the country to "shelter in place" due to "ongoing political uncertainty". Prolonged gunfire erupted near Mugabe's private residence in the suburb of Borrowdale early Wednesday, a witness told AFP. No further details were available. The armoured vehicles spotted outside Harare also alarmed residents as Chiwenga had warned of possible military intervention. The army's spokesman was not available to comment. "We very rarely see tanks on the roads," Derek Matyszak, an analyst at the Pretoria-based Institute for Security Studies, told AFP. "Chiwenga threw down the gauntlet to Mugabe... it would make sense for Chiwenga to organise some military manoeuvres to up the ante. "It's clear we are entering new territory here." - Mugabe under pressure - Mugabe is the world's oldest head of state, but his poor health has fuelled a bitter succession battle as potential replacements jockey for position. Some of the army top brass are seen as strongly opposed to Grace Mugabe's apparent emergence as the likely next president. In speeches this year, Mugabe has often slurred his words, mumbled and paused for long periods. His lengthy rule has been marked by brutal repression of dissent, mass emigration, vote-rigging and economic collapse since land reforms in 2000. The main opposition MDC party called for civilian rule to be protected. "No one wants to see a coup... If the army takes over that will be undesirable. It will bring democracy to a halt," shadow defence minister Gift Chimanikire, told AFP. Speculation has been rife in Harare that Mugabe could seek to remove Chiwenga, who is seen as an ally of ousted Mnangagwa. The crisis marks an "ominous moment in the ongoing race to succeed" Mugabe, said political analyst Alex Magaisa in an online article. "(Mugabe) has previously warned the military to stay away from ZANU-PF's succession race. "His authority over the military has never been tested in this way." Mnangagwa, 75, was widely viewed as Mugabe's most loyal lieutenant, having worked alongside him for decades. He fled the country and is thought to be in South Africa. Earlier this year the country was gripped by a bizarre spat between Grace and Mnangagwa that included an alleged ice-cream poisoning incident that laid bare the pair's rivalry. Grace Mugabe -- 41 years younger than her husband -- has become increasingly active in public life in what many say is a process to help her eventually take the top job.