(Reuters) – Democratic Republic of Congo’s army has been engaged in heavy fighting since the weekend against the M23 rebel group, which is waging its most sustained offensive since a 2012-2013 insurrection that captured vast swathes of territory. The rebels control positions as close as 20 km (12 miles) to eastern Congo’s main city of Goma and briefly took over the main military base in the province, local officials said. The fighting has forced tens of thousands to flee their homes in a region that has had little respite from conflict ever since neighbours Rwanda and Uganda invaded in 1996, c…