Pastor Tim Keller says there has been some "encouraging" news in his ongoing fight against cancer. 

The bestselling Christian writer and theologian announced in June that he had been diagnosed with pancreatic cancer. 

He has since been undergoing treatment and in a new update on Twitter, said he has just completed his sixth round of chemotherapy. 

He shared that while there have been "some side effects" to the treatment, he has been "tolerating" them "very well". 

"I have not been seriously debilitated and I can still do some work and ministry," he said. 

"Yesterday we also met with my oncologist to go over the scans taken on Monday to assess the effectiveness of the chemotherapy.

"The report is very encouraging. There has been shrinkage of the tumors and so we are continuing the chemo in order to diminish the cancer further."

Keller is already a cancer survivor, having been diagnosed with thyroid cancer in 2002. 

He has asked supporters to continue praying for him, especially that the side effects will not be too severe. 

Remarking on his faith during this season, Keller said he was experiencing God's presence in new ways. 

"Our situation has driven us to seek God's face as never before," he said.

"He is giving us more of his sensed presence, more freedom from our besetting sins, more dependence on his Word — things that we had sought for years, but only under these circumstances are we finding them."