KABUL, June 22 (Reuters) - Taliban insurgents have conducted a wave of offensives in Afghanistan"s north in recent days, moving beyond their southern strongholds as international forces withdraw. The United Nations" envoy for Afghanistan said the Taliban had taken more than 50 of 370 districts and was positioned to take control of provincial capitals. Fierce fighting between the Taliban and Afghan government forces has taken place on the outskirts of three provincial capitals in the northern provinces of Faryab, Balkh and Kunduz provinces in recent days, officials said. Since the United States announced plans in April to withdraw its troops with no conditions by Sept. 11 after nearly 20 years of conflict, violence has escalated throughout the country as the Taliban seeks more territory. Peace talks in Doha have largely stalled, officials say, though there have been meetings in recent days and the Taliban say they are committed to talks. The U.N. Secretary-General’s Special Representative for Afghanistan Deborah Lyons urged the Security Council to do all it could to push the parties back to the negotiating table. "Increased conflict in Afghanistan means increased insecurity for many other countries, near and far," she said. The latest surge in the north is outside the Taliban"s traditional strongholds in southern districts such as Helmand and Kandahar where major fighting had previously taken place. "The Taliban"s strategy is to make inroads and have a strong presence in the northern region of the country that long resisted the insurgent group," said a senior Afghan security official on condition of anonymity. "They would face less resistance in other parts of the country where they have more influence and presence." Local officials in Kunduz said the Taliban on Tuesday seized Shir Khan port, a commercial local town situated on the border between Afghanistan and Tajikistan. Ghulam Rabbani, a provincial council member, said fighting was also ongoing outside Kunduz"s provincial capital and people were fleeing the city. The defence ministry said Afghan forces had recaptured key districts from the Taliban in Kunduz and operations were ongoing. Local officials and Taliban members said the Taliban had reached the outskirts of Mazar-i-Sharif, Balkh"s capital, on Monday evening before retreating. Taliban spokesman Zabihullah Mujahid said the group"s leadership had asked fighters to return after they reached the entrance of Mazar-i-Sharif as their top leadership did not want to seize provinces until all U.S. forces had left. The United States began withdrawing troops on May 1 and has handed some bases over to the Afghan government, which has since given up some areas to the Taliban without a fight. The government admits the Taliban have captured a number of districts and security forces have made "tactical retreats". The crumbling morale of Afghan forces has raised fears of a Taliban military take over once the withdrawal of foreign forces is complete. The security official said the government was not abandoning areas to the Taliban, but was retreating from some districts temporarily for tactical reasons as they sought to preserve stretched resources. "Fighting has fiercely increased in recent weeks and now our main focus is to hold strategic areas and not to further stretch our forces," he said.