His remarks come a day after President Barack Obama also said America was not winning in Afghanistan.
And David Miliband, the foreign secretary, recently warned of a "strategic stalemate" in parts of the country.
General McKiernan said: "There are other areas – large areas in the southern Afghanistan especially but in parts of the east – where we are not winning, where more has to happen along multiple lines of operation in order for anybody by any metric to say that the Afghans are winning or the efforts of the coalition are winning."
General McKiernan has asked for tens of thousands of reinforcements and President Obama has ordered 17,000 more US soldiers to the country.
President Obama has also opened the door to talks with moderate elements of the Taliban.
Commanders have described the situation in the southern province of Helmand Province, where British troops have been fighting for nearly three years, as "stalemate".
Thinly stretched British forces hold of string of district centres, but much of the countryside remains too dangerous for government officials or aid workers.
A deadly insurgency campaign of roadside bombings has seen the British suffer their deadliest 12 months since the Afghan campaign began.
A combat brigade of several thousand US marines is due to arrive in Helmand this summer and fighting is expected to intensify further.