SANGIN (AFP) - US Defense Secretary Robert Gates touted progress on a visit to southern Afghanistan Tuesday but said NATO forces will soon face an "acid test" as Taliban insurgents move to seize back ground. In a tour of pivotal battlegrounds, Gates said he came away encouraged that coalition and Afghan forces were rolling back the Taliban from longstanding strongholds in Kandahar and Helmand provinces. But he added that the results of the NATO-led campaign would not be clear until later this year, when the Taliban is expected to exploit warmer weather to strike back. Gates's visit comes before foreign troops are due to start limited withdrawals from July, handing control of security to Afghan forces in some areas ahead of a full transition across Afghanistan due by 2014. Elsewhere, Afghan President Hamid Karzai warned Tuesday the year ahead was "critical" and would hold "crises and difficulties." "The fight this spring and summer is going to be very tough. We expect the Taliban to try and take back much of what they've lost," Gates told reporters at a US outpost in the Arghandab district, west of Kandahar city. "And that will really in many respects be the acid test about how effective the progress that we've made is." In neighbouring Helmand province, Gates earlier told US Marines they had made a "strategic breakthrough" in the district of Sangin, a Taliban bastion that has long frustrated coalition forces. "Before you arrived here the Taliban were dug in deep," said Gates, standing near a monument honouring British troops who suffered heavy losses before handing over to the Americans in October. The Pentagon chief told members of a Marine Corps battalion that he had come to see first-hand "the dramatic turnaround that you have brought about here in Sangin." Lieutenant Colonel Jason Morris of the 3rd Battalion, 5th Marine Regiment said the Taliban had been pushed off the area's main road and no longer dominated the town, a key hub for the insurgents' opium trade and supply network. But he warned violence would probably "pick up in the next couple of weeks." The tentative gains in Sangin have come at a high price for the Americans, with 29 killed and 175 wounded, the worst casualties of any battalion in the war, Gates said. During four years in Sangin district, the British lost more than 100 soldiers, accounting for nearly a third of all Britain's war dead in the conflict. Gates said plans were on track to start a gradual withdrawal of US troops in July, but he said it was unlikely the drawdown would start in the hotly contested south. He later flew west to a combat outpost in the Arghandab district in Kandahar province, where he walked through a dusty village once controlled by the Taliban. The Pentagon chief met Afghan recruits to a new local police initiative -- the brainchild of US special forces -- and spoke with village elders, some of whom used to be allied with the Taliban. The buildup of US troops combined with Afghan army and police was starting bear fruit, he said after his walking tour. "I do feel like the pieces are coming together," Gates said. The US-led fighting coalition views the southern provinces of Helmand and Kandahar, traditional Taliban heartlands bordering Pakistan's tribal areas, as crucial to turning the tide in the nine-year war. Around 20,000 marines have been deployed in Helmand alone as part of a wider surge announced by President Barack Obama in December 2009. As Gates offered an upbeat assessment, Karzai spoke of a "critical" year ahead for the fight against the Taliban during a speech in Kabul. "This is extremely important and critical," he said. "This is a year in which we will face crises and difficulties... the success that we hope for depends on our unity and statesmanship." Karzai is due to give details on March 21 of which areas of Afghanistan will be the first to make the transition. He offered a teaser on his plans, saying "five to six places" would be handed over to Afghan forces in the first stage, but did not specify whether these were towns, districts or provinces.